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We present theoretical and experimental analysis of photocarrier kinetics in quantum dot (QD) solar cells. The measurements of the J–V characteristics reveal strong effects of QD charging by selective doping of the interdot space on the solar cell characteristics. We demonstrate that charging of QDs significantly increases electron coupling to sub-bandgap photons, provides effective harvesting of IR energy, and serve as an effective tool for manipulating the potential profile at the micro- and nanoscale. The potential well for electrons in InAs QDs is substantially deeper than that for holes and, due to major differences between the effective masses of electrons and holes, the electron level spacing is substantially larger than the level spacing for holes. Therefore, QDs act as deep traps for electrons but shallow traps for holes. Filling of QDs under illumination is determined by a condition of equality of electron and hole capture rates which is realized via strong exponential dependence of the capture rates on the potential barrier around a charged dot. Without adequate doping of the QD medium, QDs are filled by electrons from the n-doped junction area and deteriorate the solar cell performance. However, selective n-doping of the QD medium results in micro- and nanoscale potential profiles favorable for photovoltaic conversion. Potential barriers around charged QDs decrease the photoelectron capture processes and suppress recombination processes via QDs. The filling of QDs predominantly from dopants in the QD medium allows one to maintain the microscale potential profile analogous to that in the best conventional single-junction solar cells.

The lysogenic state of phage λ is maintained by the CI repressor. CI binds to three operators each in the right operator ( O R ) and left operator ( O L ) regions, which lie 2.4 kb apart. At moderate CI levels, the predominant binding pattern is two dimers of CI bound cooperatively at each regulatory region. The resulting tetramers can then interact, forming an octamer and a loop of the intervening DNA. CI is expressed from the P RM promoter, which lies in the O R region and is subjected to multiple regulatory controls. Of these, the most recently discovered is stimulation by loop formation. In this work, we have investigated the mechanism by which looping stimulates P RM . We find that two cis -acting sites lying in the O L region are involved. One site, an UP element, is required for stimulation. Based on the behavior of other promoters with UP elements located upstream of the −35 region, we suggest that a subunit of RNA polymerase (RNAP) bound at P RM binds to the UP element located in the O L region. In addition, adjacent to the UP element lies a binding site for integration host factor (IHF); this site plays a less critical role but is required for stimulation of the weak prm240 allele. A loop with CI at the O L 2 and O L 3 operators does not stimulate P RM , while one with CI only at O L 2 provides some stimulation. We discuss possible mechanisms for stimulation.

We present computational and experimental results of dust particles that can be tuned to preferentially reflect or emit IR radiation within the 8–14 μm band. The particles consist of thin metallic subwavelength gratings patterned on the surface of a simple quarter wavelength cavity. This design creates distinct IR absorption resonances by combining the plasmonic resonance of the grating with the natural resonance of the cavity. We show that the resonance peaks are easily tuned by varying either the geometry of the grating or the thickness of the cavity. Here, we present a computational design algorithm along with experimental results that validate the design methodology.

The gene regulatory circuitry of phage lambda is among the best-understood circuits. Much of the circuitry centres around the immunity region, which includes genes for two repressors, CI and Cro, and their cis-acting sites. Related phages, termed lambdoid phages, have different immunity regions, but similar regulatory circuitry and genome organization to that of lambda, and show a mosaic organization, arising by recombination between lambdoid phages. We sequenced the immunity regions of several wild phages with the immunity specificity of lambda, both to determine whether natural variation exists in regulation, and to analyse conservation and variability in a region rich in well-studied regulatory elements. CI, Cro and their cis-acting sites are almost identical to those in lambda, implying that regulatory mechanisms controlled by the immunity region are conserved. A segment adjacent to one of the operator regions is also conserved, and may be a novel regulatory element. In most isolates, different alleles of two regulatory proteins (N and CII) flank the immunity region; possibly the lysis-lysogeny decision is more variable among isolates. Extensive mosaicism was observed for several elements flanking the immunity region. Very short sequence elements or microhomologies were also identified. Our findings suggest mechanisms by which fine-scale mosaicism arises.

Analysis of synthetic gene regulatory circuits can provide insight into circuit behavior and evolution. An alternative approach is to modify a naturally occurring circuit, by using genetic methods to select functional circuits and evolve their properties. We have applied this approach to the circuitry of phage lambda. This phage grows lytically, forms stable lysogens, and can switch from this regulatory state to lytic growth. Genetic selections are available for each behavior. We previously replaced lambda Cro in the intact phage with a module including Lac repressor, whose function is tunable with small molecules, and several cis-acting sites. Here, we have in addition replaced lambda CI repressor with another tunable module, Tet repressor and several cis-acting sites. Tet repressor lacks several important properties of CI, including positive autoregulation and cooperative DNA binding. Using a combinatorial approach, we isolated phage variants with behavior similar to that of WT lambda. These variants grew lytically and formed stable lysogens. Lysogens underwent prophage induction upon addition of a ligand that weakens binding by the Tet repressor. Strikingly, however, addition of a ligand that weakens binding by Lac repressor also induced lysogens. This finding indicates that Lac repressor was present in the lysogens and was necessary for stable lysogeny. Therefore, these isolates had an altered wiring diagram from that of lambda. We speculate that this complexity is needed to compensate for the missing features. Our method is generally useful for making customized gene regulatory circuits whose activity is regulated by small molecules or protein cofactors.

Bistable gene regulatory circuits can adopt more than one stable epigenetic state. To understand how natural circuits have this and other systems properties, several groups have designed regulatory circuits de novo. Here we describe an alternative approach. We have modified an existing bistable circuit, that of phage λ. With this approach, we used powerful genetic selections to identify functional circuits and selected for variants with altered behavior. The λ circuit involves two antagonistic repressors, CI and Cro. We replaced λ Cro with a module that included Lac repressor and several lac operators. Using a combinatorial approach, we isolated variants with different types of regulatory behavior. Several resembled wild-type λ—they could grow lytically, could form highly stable lysogens, and carried out prophage induction. Another variant could form stable lysogens in the presence of a ligand for Lac repressor but switched to the lytic state when the ligand was removed. Several isolates evolved toward a desired behavior under selective pressure. These results strongly support the idea that complex circuits can arise during the course of evolution by a combination of simpler regulatory modules. They also underscore the advantages of modifying a natural circuit as an approach to understanding circuit design, systems behavior, and circuit evolution.

For the LBT Observatory, the next couple of years promise to be both exciting and challenging. Exciting as the long awaited suite of first generation instruments and GLAO become available for binocular operations, while regular interferometric observations will make LBT the first operational ELT. Challenging because LBTO will have to handle maintenance and upgrades of instruments or key components like its adaptive secondaries about which it has much to learn. Step1 will outline a plan optimizing LBTOs scientific production while mitigating the consequences of the inevitable setbacks the challenges will bring.

Nanoscale engineering of band profile and potential profile provide effective tools for the management of photoelectron processes in quantum dot (QD) photovoltaic devices. We investigate the QD devices with various 1-μm InAs /GaAs QD media placed in a 3-μm base GaAs p-n junction. We found that n-charging of quantum dots (QDs) create potential barriers around QDs. QD growth between ultrathin AlGaAs layers leads to the formation of AlGaAs “fence” barriers, and reduces the wetting layers (WLs). The barriers around QDs and reduction of the wetting layer substantially suppress recombination processes via QDs. The n-doping of interdot space in QD media enhances electron extraction from QDs. All of our QD devices show short-circuit current, J SC , higher than that of the reference cell, but smaller open-circuit voltage, V OC .. In the QD devices, the short circuit currents increase by ~0.1 mA/cm 2 per dot layer. J SC reaches 28.4 mA/cm 2 in the device with QD media that combines dot charging, fence barriers, and WL reduction.

Temperate phages can carry genes that affect the phenotype and behavior of their bacterial host. These genes can be considered extra genetic material in that they are not necessary for viral lytic growth or for the lysogenic lifestyle. This chapter describes circumstances under which foreign genes can be expressed. It first considers the life cycle of temperate phages. With this background, it then describes how foreign genes can be expressed in the lysogenic state. It then turns to a more detailed description of a particular temperate phage λ, with an emphasis on the regulatory mechanisms that are best understood for this phage. This description will facilitate an understanding of how foreign genes can be expressed during the process of prophage induction, and a particular example will be described. Importantly, for lysogenic conversion to occur, it is not necessary that the prophage remains functional as a virus that is capable of prophage induction or lytic growth. Foreign genes are most commonly expressed from a prophage in the lysogenic state. This pattern of expression is often termed lysogenic conversion, since it can change or convert the phenotype of the bacterial host. The chapter also reviews the gene organization of phage λ and then briefly describes the pattern of expression of these genes during lytic growth. RecA plays a central role in homologous recombination and DNA repair, catalyzing a complex set of DNA strand transfer reactions.

LexA repressor undergoes a self-cleavage reaction. In vivo, this reaction requires an activated form of RecA, but it occurs spontaneously in vitro at high pH. Accordingly, LexA must both allow self-cleavage and yet prevent this reaction in the absence of a stimulus. We have solved the crystal structures of several mutant forms of LexA. Strikingly, two distinct conformations are observed, one compatible with cleavage, and the other in which the cleavage site is approximately 20 A from the catalytic center. Our analysis provides insight into the structural and energetic features that modulate the interconversion between these two forms and hence the rate of the self-cleavage reaction. We suggest RecA activates the self-cleavage of LexA and related proteins through selective stabilization of the cleavable conformation.

Complex interacting systems exhibit system behavior that is often not predictable from the properties of the component parts. We have tested a particular system property, that of robustness. The behavior of a system is termed robust if that behavior is qualitatively normal in the face of substantial changes to the system components. Here we test whether the behavior of the phage lambda gene regulatory circuitry is robust. This circuitry can exist in two alternative patterns of gene expression, and can switch from one regulatory state to the other. These states are stabilized by the action at the O(R) region of two regulatory proteins, CI and Cro, which bind with differential affinities to the O(R)1 and O(R)3 sites, such that each represses the synthesis of the other one. In this work, this pattern of binding was altered by making three mutant phages in which O(R)1 and O(R)3 were identical. These variants had the same qualitative in vivo patterns of gene expression as wild type. We conclude that the behavior of the lambda circuitry is highly robust. Based on these and other results, we propose a two-step pathway, in which robustness plays a key role, for evolution of complex regulatory circuitry.

The CI repressors of lambdoid phages bind cooperatively to adjacent binding sites. Although these binding sites are found at complex operators containing three binding sites, cooperative binding occurs only between dimers bound at two of the sites, a mode of binding termed pairwise cooperativity. At the level of regulation, pairwise cooperativity allows the proper operation of the genetic switch. At the mechanistic level, it has been proposed to result from steric distortion of the complex, such that a protein dimer bound to a central site cannot contact both flanking dimers because it “leans” towards one of the two sites. We have tested this model using the CI repressor of phage HK022. Previous work suggested that reducing the spacing between adjacent operators might allow cooperative interaction among three dimers, a mode of cooperativity we term extended. Using a set of synthetic templates, we have shown that reducing the spacing allows extended cooperativity among three binding sites. Accordingly, the mode of cooperativity changes qualitatively in response to changes in the inter-site spacing. That is, the change in spacing has major functional consequences for the properties of the complex. We suggest that such changes can play important roles in the evolution of gene regulatory circuitry and of other processes involving nucleoprotein complexes.

Cooperative protein-DNA interactions play critical roles in gene regulation in all organisms. Among the best-studied cooperative interactions is that of phage λ repressor, which binds cooperatively to two adjacent operators. Similar cooperative interactions are also shown by several other lambdoid phage repressors, including HK022 CI repressor, which we study here. This protein has a much higher degree of cooperativity than seen with λ repressor, and previous evidence has suggested that cooperativity may play roles in HK022 gene regulation that have no parallel in λ. We have isolated several cooperativity or Coop− mutations in HK022 cI. These mutant proteins were partially defective in vivo for binding to two adjacent operators, but normal or nearly so for binding to a single operator. Two mutations showed mutual suppression, in that the double mutation had wild-type behavior. Analysis of several purified mutant proteins showed that they were also defective for cooperative binding in vitro. Unexpectedly, the mutant proteins showed an altered pattern of in vitro binding to DNA at non-operator sites. Several of them also increased the rate of specific repressor cleavage. We propose a conformational model in which the various functions of the wild-type protein are carried out by differing conformations; these conformations are normally in balance, and the mutations perturb this balance.

In self-processing biochemical reactions, a protein or RNA molecule specifically modifies its own structure. Many such reactions are regulated in response to the needs of the cell by an interaction with another effector molecule. In the system we study here, specific cleavage of the Escherichia coli LexA repressor, LexA cleaves itself in vitro at a slow rate, but in vivo cleavage requires interaction with an activated form of RecA protein. RecA acts indirectly as a coprotease to stimulate LexA autodigestion. We describe here a new class of lexA mutants, lexA (Adg-; for autodigestion-defective) mutants, termed Adg- for brevity. Adg- mutants specifically interfered with the ability of LexA to autodigest but left intact its ability to undergo RecA-mediated cleavage. The data are consistent with a conformational model in which RecA favors a reactive conformation capable of undergoing cleavage. To our knowledge, this is the first example of a mutation in a regulated self-processing reaction that impairs the rate of self-processing without markedly affecting the stimulated reaction. Had wild-type lexA carried such a substitution, discovery of its self-processing would have been difficult; we suggest that, in other systems, a slow rate of self-processing has prevented recognition that a reaction is of this nature.

We have designed and characterized an infrared spectrometer, which uses a linear array of quantum grid infrared photodetectors (QGIPs) as its spectral sensing elements. Each QGIP element shares the same detector material but has a different grid geometry. The detector material, which is based on a binary superlattice design, provides an 8–14 μm broadband absorption medium for the spectrometer. The geometry of the grid, which is the light coupling structure under normal incidence, selects individual absorption wavelength for each element. Using a linear array of QGIPs of different geometries, multiple wavelengths can be detected simultaneously, and the array thus forms a spectrometer. Multicolor infrared imaging can then be achieved by integrating such QGIPs in unit cells of a two-dimensional array.

We report on the realization of a GaAs type-I quantum dot solar cell with a record 29.9 mA/cm2 short circuit current density and we compare it to a reference bulk GaAs solar cell.

Novel approach to control of photoelectron processes is based on nanoscale engineering of 3D potential profile employing quantum dots with built-in charge (Q-BIC). Charging of quantum dots creates local potential barriers around single dots, if dots are arbitrary distributed in the medium, and collective barriers around dot clusters, rows etc, if quantum dots form specific structures. Manipulations with potential barriers provide an effective tool for suppression of fast capture processes of photocarriers by quantum dots. This allows one to increase the photocarrier lifetime and to reduce the recombination losses. The charging of dots also enhances the electron coupling to infrared radiation and multi-step absorption of sub-gap photons. Q-BIC nanomaterials have a number of attractive features to be used in photovoltaic and sensing applications.

We present theoretical and experimental results of electron kinetics and transport in quantum dot structures with potential barriers created around dots via intentional or unintentional doping. Monte Carlo simulations demonstrate that photoelectron capture is substantially enhanced in strong fields and electron kinetics can be controlled by potential barriers. Therefore, by creating potential barriers around dots, we found that our novel quantum dots with built-in charge (Q-BIC) solar cells and infrared (IR) photodectors enhance electron intersubband transitions and suppress fast electron capture processes. These factors lead to a 60% increase in the photocurrent of the Q-BIC solar cells (without degradation of the open circuit voltage) and ~25 times increase in the photoresponse of the Q-BIC photodetectors.

Cooperative binding of specific DNA-binding proteins plays crucial roles in gene regulatory circuitry, and is a model system for interactions between proteins bound to DNA. We have studied coliphage HK022 repressor, which binds to two adjacent operators with a cooperativity parameter of ≈ 2000. We examined the effect of changing the spacing between these two operators on cooperativity and on the conformation of the complex. Maximum cooperativity was seen with the wild-type spacing; considerable cooperativity was retained for most spacing variants, but was abolished when the operators lay on opposite faces of the DNA helix. Most spacing variants conferred changes in the conformation of the DNA-protein complex. Our data indicate that the pairwise cooperativity observed with the wild-type spacing results from a conformation that prevents protein-protein contacts with flanking bound dimers. We conclude that protein-DNA complexes involving the same specific binding sites and the same protein molecules can adopt many different conformations, depending on the spacing between the binding sites. This conclusion may be broadly applicable to protein-DNA interactions in other systems.

This report describes the highlights and summary of the year 2, quarter four (Y2Q4) progress for the 6.2 project Improved Sensitivity Low-Cost Uncooled IR Detector Focal-Plane Arrays. This work occurred during the January 2010 May 2010 time period. The program goals for Y2Q4 corresponded to the continued expansion of our barium titanium oxide (BaTiO3) nanomaterial synthesis capabilities and deposition methods; laser-induced pyroelectric phase conversion studies and nanoscale characterization of the pyroelectric activation process; and fabrication of infrared (IR) test heterostructures for pyroelectrical analysis, along with the development of a technique for generating the requisite IR absorbing layer.

We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm 2 compared with 15.07 mA/cm 2 in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.

Diminished performance due to poor chemical and electrical surface stability of InAs/GaSb SL photodetectors continues to be a major hurdle to the realization of the theoretically predicted high performance of this material system. Improved epitaxial growth conditions have yielded improvements in material quality over the past several years. However, surface instability resulting in electrical shunt pathways across the junction, and diminished device performance over time is still a major limiting factor for application of InAs/GaSb SL in long-wavelength infrared detectors. This study focuses on a two-step approach towards the successful surface passivation of long-wavelength InAs/GaSb superlattice structures. Two distinct sulfide chemical surface treatments were applied to inhibit the formation of native surface oxides and satisfy dangling bonds. This was followed by the application of a robust SU8-2 dielectric treatment on the mesa sidewalls to inhibit sulfide layer degradation and oxidation of the surface over time. A variable area diode analysis (VADA) technique employing diodes of variable diameter (40-400um) enabled the investigation of surface resistivity as a result of different passivation treatments. Temperature dependent studies of the dark current were used to understand the dominating current mechanisms.

This report describes the highlights and summary of the year two, quarters two and three progress for the 6.2 project Improved Sensitivity Low-Cost Uncooled IR Detector Focal-Plane Arrays. This work occurred during the August 2009 January 2010 time period. The program goals for year 2-quarters 2&3 corresponded to the continued expansion of our BaTiO3 nanomaterial synthesis capabilities and deposition methods, laser-induced pyroelectric phase conversion studies and nanoscale characterization of the pyroelectric activation process, and fabrication of IR test heterostructures for pyroelectrical analysis, along with the development of a technique for generating the requisite IR absorbing layer.

We report a 50% increase in power efficiency for our novel InAs/GaAs quantum dot with built-in charge (Q-BIC) solar cells. We found that n-doping the inter-dot space of a quantum dot solar cell (QDoSC) increases the short circuit current density from 15.07 mA/cm2 in undoped QDoSC to 24.30 mA/cm2 in the device doped to provide approximately six electrons per dot. To identify the physical mechanisms that provide this significant improvement, we investigate the photovoltaic response and its spectral characteristics in GaAs reference cell, undoped, n-doped, and p-doped QDoSCs. We found that the photovoltaic efficiency of the undoped QDoSC is almost the same as that of the reference cell. The efficiency monotonically improves with increasing n-doping, while the p-doping deteriorates the photovoltaic conversion. Studies of the photoluminescence of p- and n-doping show that the photoelectron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The built-in-dot electron charge enhances electron inter-subband QD transitions, suppresses the fast electron capture processes, and together with charged donors, forms the potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and to a radical improvement of the QDoSC efficiency. Even higher efficiencies are anticipated for higher n-doping levels.

In this paper we present computational and experimental results of dust particles that can be tuned to preferentially reflect or emit IR radiation within the 8-14 μm band. The particles consist of thin metallic subwavelength gratings patterned on the surface of a simple quarter wavelength cavity. This design creates distinct IR absorption resonances by combining the plasmonic resonance of the grating with the natural resonance of the cavity. We will show that the resonance peaks are easily tuned by varying either the geometry of the grating or the thickness of the cavity. Here, we present a computational design algorithm along with experimental results that validate the design methodology.

The recA432 mutant allele was isolated (T. Kato and Y. Shinoura, Mol. Gen. Genet. 156:121-131, 1977) by virtue of its defect in cellular mutagenesis (Mut-) and its hypersensitivity to damage by UV irradiation (UVs), which were phenotypes expected for a recA mutant. However, we found that in a different genetic background (lexA51 sulA211 uvrB+), recA432 mutants expressed certain mutant phenotypes but not the Mut- and UVs phenotypes (D.G. Ennis, N. Ossanna, and D.W. Mount, J. Bacteriol. 171:2533-2541, 1989). We present several lines of evidence that these differences resulted from the sulA genotype of the cell and that the apparent UVs and Mut- phenotypes of the sulA+ derivatives resulted from lethal filamentation of induced cells because of persistent derepression of sulA. First, transduction of sulA(Def) mutations into the recA432 strains restored cellular mutagenesis and resistance to UV. Second, recA432 sulA+ strains underwent filamentous death following SOS-inducing treatments. Third, cleavage of LexA repressor in a recA432 strain continued at a rapid rate long after UV induction, at a time when cleavage of the repressor in the recA+ parental strain had substantially declined. Fourth, we confirmed that a single mutation (recA432) conferring both the UVs and Mut- phenotypes mapped to the recA gene. These findings indicate that the RecA432 mutant protein is defective in making the transition back to the deactivated state following SOS induction; thus, the SOS-induced state of recA432 mutants is prolonged and can account for an excess of SulA protein, leading to filamentation. These results are discussed in the context of molecular models for RecA activation for LexA and UmuD cleavage and their roles in the control of mutagenesis and cell division in the SOS response.

The program goals for year 2-quarter 1 (Y2Q1) Institute for Collaborative Biotechnologies project 'Lower Cost Uncooled IR Detector Focal Plane Arrays' included (1) the continued expansion of our barium titanium oxide (BaTiO3) nanomaterial synthesis capabilities and deposition methods, (2) laser-induced pyroelectric phase conversion studies and spectroscopic end-point control scheme, and (3) comprehensive material characterization studies, along with electrical property characterization of BaTiO3 thin films. This report covers progress that took place in the three month period from May 1, 2009-July 30, 2009.

Complex gene regulatory circuits exhibit emergent properties that are difficult to predict from the behavior of the components. One such property is the stability of regulatory states. Here we analyze the stability of the lysogenic state of phage λ. In this state, the virus maintains a stable association with the host, and the lytic functions of the virus are repressed by the viral CI repressor. This state readily switches to the lytic pathway when the host SOS system is induced. A low level of SOS-dependent switching occurs without an overt stimulus. We found that the intrinsic rate of switching to the lytic pathway, measured in a host lacking the SOS response, was almost undetectably low, probably less than 10 −8 /generation. We surmise that this low rate has not been selected directly during evolution but results from optimizing the rate of switching in a wild-type host over the natural range of SOS-inducing conditions. We also analyzed a mutant, λ prm240 , in which the promoter controlling CI expression was weakened, rendering lysogens unstable. Strikingly, the intrinsic stability of λ prm240 lysogens depended markedly on the growth conditions; lysogens grown in minimal medium were nearly stable but switched at high rates when grown in rich medium. These effects on stability likely reflect corresponding effects on the strength of the prm240 promoter, measured in an uncoupled assay system. Several derivatives of λ prm240 with altered stabilities were characterized. This mutant and its derivatives afford a model system for further analysis of stability.

Characterisation, mitigation and correction of telescope vibrations have proven to be crucial for the performance of astronomical infrared interferometers. The project teams of the interferometers for the LBT, LINC-NIRVANA and LBTI, and LBT Observatory (LBTO) have embarked on a joint effort to implement an accelerometer-based vibration measurement system distributed over the optical elements of the LBT. OVMS, the Optical Path Difference and Vibration Monitoring System will serve to (i) ensure conditions suitable for adaptive optics (AO) and interferometric (IF) observations and (ii) utilize vibration information, converted into tip-tilt and optical path difference data, in the control strategies of the LBT adaptive secondary mirrors and the beam combining interferometers. The system hardware is mainly developed by Steward Observatory's LBTI team and its installation at the LBT is underway. The OVMS software development and associated computer infrastructure is the responsibility of the LINC-NIRVANA team at MPIA Heidelberg. Initially, the OVMS will fill a data archive provided by LBTO that will be used to study vibration data and correlate them with telescope movements and environmental parameters thereby identifiying sources of vibrations and to eliminate or mitigate them. Data display tools will help LBTO staff to keep vibrations within predefined thresholds for quiet conditions for AO and IF observations. Later-on real-time data from the OVMS will be fed into the control loops of the AO systems and IF instruments in order to permit the correction of vibration signals with frequencies up to 450 Hz.

Specific cleavage of LexA repressor plays a crucial role in the SOS response of Escherichia coli. In vivo, cleavage requires an activated form of RecA protein. However, previous work has shown that the mechanism of cleavage is unusual, in that the chemistry of cleavage is probably carried out by residues in the repressor, and not those in RecA; RecA appears to facilitate this reaction, acting as a coprotease. We recently described a new type of lexA mutation, a class termed lexA (IndS) and here called IndS, that confers an increased rate of in vivo cleavage. Here, we have characterized the in vitro cleavage of these IndS mutant proteins, and of several double mutant proteins containing an IndS mutation and one of several mutations, termed Ind-, that decrease the rate of cleavage. We found, first, that the autodigestion reaction for the IndS mutant proteins had a higher maximum rate and a lower apparent pKa than wild-type LexA. Second, the IndS mutations had little or no effect on the rate of RecA-mediated cleavage, measured at low protein concentrations, implying that the value of Kcat/Km was unaffected. Third, the rate of autodigestion for the double-mutant proteins, relative to wild-type, was about that rate predicted from the product of the effects of the two single mutations. Finally, by contrast, these proteins displayed the same rate of RecA-mediated cleavage as did the single Ind- mutant protein. We interpret these data to mean that the IndS mutations mimic to some extent the effect of RecA on cleavage, perhaps by favoring a conformational change in LexA. We present and analyze a model that embodies these conclusions.

Infrared (IR) photon detectors based on III-V semiconductor quantum structures such as type-II superlattices [1] and quantum well infrared photodetectors (QWIPs) [2] require atomically flat interfaces, which are achieved through expensive, high-vacuum growth techniques such as molecular beam epitaxy (MBE). These structures are fabricated into detector arrays which are bump-bonded to Silicon (Si) Read-Out Integrated Circuits (ROICs). IR detector applications involving high resolution and fast refresh rates use highly sensitive photon detectors which require external cooling; making them bulky, expensive, and therefore impractical for equipping each soldier on the battlefield [3]. Uncooled thermal detectors tend to be lower cost, but the challenge is in developing materials that are thermally compatible with Si readout electronics. Thermal detectors convert incident radiation to heat. When a pyroelectric material is subjected to a temperature change, a dipole is induced, which sets up an electric potential. The resulting pyroelectric current can be read by the Si electronics. IR detectors based on perovskite oxides such as Barium Titanate (BaTiO 3 ) are of interest in part because of their lack of need for cryogenic cooling, which makes them relatively more affordable and operationally simpler than cooled photon detector systems. Using a biologically-inspired low-temperature nanoparticle deposition technique, direct-write laser phase conversion, and micro-electro-mechanical systems (MEMS) fabrication techniques, we are working towards an uncooled IR focal plane array (FPA) process compatible with monolithic integration of the detector pixels directly onto ROICs.

Pyroelectric infrared (IR) detectors based on perovskite oxides are of interest in part because of their lack of need for cooling, which makes them relatively more affordable and operationally simpler than cooled photon detector systems. We are investigating two methods for low-cost growth of perovskite oxide thin films, namely, a bio-inspired, low-temperature synthesis method and a modified industry-standard metalorganic solution deposition (MOSD) method. Subsequent to film synthesis, we utilize direct-write laser phase conversion and micro-electro-mechanical systems (MEMS) fabrication for development of an uncooled IR focal plane array (FPA). Film growth, crystallization and MEMS processes are compatible with monolithic integration of the detector pixels directly onto Si readout integrated circuits (ROICs).

Specific LexA cleavage can occur under two different conditions: RecA-mediated cleavage requires an activated form of RecA, while an intramolecular self-cleavage termed autodigestion proceeds spontaneously at high pH and does not involve RecA. The two cleavage reactions are closely related. We postulate that RecA stimulates autodigestion rather than acting as a typical protease, and it is proposed to term this activity 'RecA coprotease' to emphasize this indirect role. The mechanism of autodigestion is similar to that of a serine protease, and RecA appears to act by reducing the pKa of a critical lysine residue LexA. A new class of mutants, termed lexA (IndS), is described; these mutations increase the rate of LexA cleavage.

At the time of immediate breast reconstruction with submuscular implants, 76 consecutive patients (89 breasts) were randomized into two groups. One received a gel-saline, double-lumen implant with 40 cc of saline added to the outer lumen, while the other received the same implant plus 40 cc of saline and 16 mg methyl prednisolone (40 mg%). Patients were followed for a minimum of 3 years. The groups, which were matched for patient age and implant size, were evaluated at 3, 12, 24, and 36 months for capsular contracture, steroid atrophy, and other complications. With completion of the double-blind study, the patients with submuscular gel-saline implants with only saline added had an overall capsular contracture rate of 38 percent at 3 months, 38 percent at 12 months, and 44 percent at 24 and 36 months. Those with methyl prednisolone had an overall capsular contracture rate of 14 percent at 3 months, and this remained unchanged through the end of the study. The rates of all other complications were comparable. Methyl prednisolone in a dose of 16 mg in 40 cc saline (concentration 40 mg%), when used in the outer lumen of a double-lumen gel-saline implant in a submuscular pocket, is both safe and efficacious in reducing the risk of capsular contracture for a minimum of 3 years in patients undergoing immediate breast reconstruction with submuscular mammary implants.

We report the results of a study of the temperature-dependent photoluminescence (PL) and leakage current characteristics of a set of type II idium arsenide (InAs)-gallium antimonide (GaSb) superlattice (SL) photodiode structures. We find that the PL efficiency of high-quality structures is determined by Shockley-Read and trap-assisted tunneling nonradiative recombination processes. Our results suggest a possible correlation between trap-assisted tunneling in some SL structures and an anomalous decrease in the PL efficiency with increasing temperature over the range 40-78 K, and provide insight into the effect of defects and SL homogeneity on the PL and transport characteristics of the photodiodes.

The LexA repressor of Escherichia coli modulates the expression of the SOS regulon. In the presence of DNA damaging agents in vivo, the 202-amino acid LexA repressor is inactivated by specific RecA-mediated cleavage of the Ala-84/Gly-85 peptide bond. In vitro. LexA cleavage requires activated RecA at neutral pH, and proceeds spontaneously at high pH in an intramolecular reaction termed autodigestion. A model has been proposed for the mechanism of autodigestion in which serine 119 serves as the reactive nucleophile that attacks the Ala-84/Gly-85 peptide bond in a manner analogous to a serine protease, while uncharged lysine 156 activates the serine 119 hydroxyl group. In this work, we have tested this model by examining the effect of the serine protease inhibitor diisopropyl fluorophosphate (DFP) on autodigestion. We found that DFP inhibited autodigestion and that serine 119 was the only serine residue to react with DFP. We also examined [3H]DFP incorporation by a number of cleavage-impaired LexA mutant proteins and found that mutations in the proposed active site, but not in the cleavage site, significantly reduced the rate of [3H]DFP incorporation. Finally, we showed that the purified carboxyl-terminal domain, which contains the proposed catalytic residues, incorporated [3H]DFP at a rate indistinguishable from the intact protein. These data further support our current model for the mechanism of autodigestion and the organization of LexA.

Complex gene regulatory circuits contain many interacting components. In principle, all of these components and interactions may be essential to the function of the circuit. Alternatively, some of them may be refinements to a simpler version of the circuit that improve its fitness. In this work, we have tested whether a particular property of a critical regulatory protein, CI, is essential to the behavior of the phage λ regulatory circuit. In the lysogenic state, CI represses the expression of the lytic genes, allowing a stable lysogenic state, by binding cooperatively to six operators. A mutant phage lacking cooperativity because of a change in c I could not form stable lysogens; however, this defect could be suppressed by the addition of mutations that altered two cis -acting sites but did not restore cooperativity. The resulting triple mutant was able to grow lytically, form stable single lysogens, and switch to lytic growth upon prophage induction, showing a threshold response in switching similar to that of wild-type λ. We conclude that cooperative DNA binding by CI is not essential for these properties of the λ circuitry, provided that suppressors increase the level of CI. Unlike wild-type lysogens, mutant lysogens were somewhat unstable under certain growth conditions. We surmise that cooperativity is a refinement to a more basic circuit, and that it affords increased stability to the lysogenic state in response to environmental variations.

A critical step in the SOS response of Escherichia coli is the specific proteolytic cleavage of the LexA repressor. This reaction is catalyzed by an activated form of RecA, acting as a co-protease to stimulate the self-cleavage activity of LexA. This process has been reexamined in light of evidence that LexA is dimeric at physiological concentrations. We found that RecA-dependent cleavage was robust under conditions in which LexA is largely dimeric and conclude that LexA dimers are cleavable. We also found that LexA dimers dissociate slowly. Furthermore, our evidence suggests that interactions between the two subunits of a LexA dimer can influence the rate of cleavage. Finally, our evidence suggests that RecA stimulates the transition of LexA from its noncleavable to its cleavable conformation and therefore operates, at least in part, by an allosteric mechanism.

Under relatively high pump intensities, photoluminescence intensities across the miniband were greatly reduced, which was attributed to the existence of periodic electric fields caused by spatially-separated photogenerated electrons and holes in short-period type-II InAs/GaSb superlattices.

Effects of concentrated solar radiation on photovoltaic performance are investigated in well-developed GaAs quantum dot (QD) solar cells with 1-Sun efficiencies of 18%–19%. In these devices, the conversion processes are enhanced by nanoscale potential barriers and/or AlGaAs atomically thin barriers around QDs, which prevent photoelectron capture to QDs. Under concentrated radiation, the short circuit current increases proportionally to the concentration and the open circuit voltage shows the logarithmic increase. In the range up to hundred Suns, the contributions of QDs to the photocurrent are proportional to the light concentration. The ideality factors of 1.1–1.3 found from the VOC-Sun characteristics demonstrate effective suppression of recombination processes in barrier-separated QDs. The conversion efficiency shows the wide maximum in the range of 40–90 Suns and reaches 21.6%. Detailed analysis of I-V-Sun characteristics shows that at low intensities, the series resistance decreases inversely proport...

The Large Binocular Telescope (LBT) features dual 8.4 m diameter mirrors in a common elevation-over-azimuth mount. The LBT moves in elevation on two large crescent-shaped C-rings that are supported by radial hydrostatic bearing pads located near the four corners of the rectangular azimuth frame. The azimuth frame, in turn, is supported by four hydrostatic bearing pads and uses hydrodynamic roller bearings for centering. Each axis is gear driven by four large electric motors. In addition to precision optical motor encoders, each axis is equipped with Farrand Inductosyn strip encoders which yield 0.005 arcsecond resolution. The telescope weighs 580 metric tons and is designed to track with 0.03 arcsecond or better servo precision under wind speeds as high as 24 km/hr. Though the telescope is still under construction, the Mount Control System (MCS) has been routinely exercised to achieve First Light. The authors present a description of the unique, DSP-based synchronous architecture of the MCS and its capabilities.

Using a module exchange approach, we have tested a long-standing model for the role of Cro repressor in λ prophage induction. This epigenetic switch from lysogeny to the lytic state occurs on activation of the host SOS system, which leads to specific cleavage of CI repressor. It has been proposed that Cro repressor, which operates during lytic growth and which we shall term the lytic repressor, is crucial to prophage induction. In this view, Cro binds to the O R 3 operator, thereby repressing the cI gene and making the switch irreversible. Here we tested this model by replacing λ Cro with a dimeric form of Lac repressor and adding several lac operators. This approach allowed us to regulate the function of the lytic repressor at will and to prevent it from repressing cI , because lac repressor could not repress P RM in our constructs. Repression of cI by the lytic repressor was not required for prophage induction to occur. However, our evidence suggests that this binding can make induction more efficient, particularly at intermediate levels of DNA damage that otherwise cause induction of only a fraction of the population. These results indicate that this strategy of module exchange will have broad applications for analysis of gene regulatory circuits.

We are investigating the structural, electrical, and infrared optical properties of a new material system comprising undoped self-assembled quantum dots having a type II band alignment with the matrix material in which they are embedded. This material system is fundamentally different from conventional type I quantum dots that must be doped and that rely on intersubband transitions for infrared photoresponse. Type II quantum dots would operate in photovoltaic mode with infrared photoresponse arising from electron-hole pair production involving 3-dimensionally-confined states in the dots and quantum well states in the matrix material. This paper emphasizes the structural characterization of molecular beam epitaxy (MBE)-grown InSb quantum dots grown in an In0.53Ga0.47As matrix lattice matched to an InP substrate.

The Type II broken band-gap alignment in semiconductor structures wherein the conduction band minimum is in one semiconductor (e.g., InAs) and the valence band maximum is in another (e.g., GaInSb) offers certain unique advantages which can be utilized to realize band-gap engineered novel quantum electro-optic devices such as lasers and detectors. The advantages of the type II structures include reduced Auger recombination, extending the effective band-gap energy of materials wherein type I band-gap alignment would give rise to difficulties such as miscibility gap. In this paper we describe the work carried out at the Army Research Laboratory on type II semiconductor quantum electro-optic devices such as IR lasers and detectors operating in the 3-12 μm range. Specifically we will cover the progress made in GaSb based type II strained layer superlattice IR detectors and Interband Cascade IR Lasers. We will also present our recent work in self-assembled quantum dots which have type II band-gap alignment with the matrix material in which the dots are embedded.

We demonstrate that longitudinal-optical phonons in type-II InAs/GaSb quantum wells can be used to up-convert photoluminescence. Such a phenomenon could be used to achieve laser-induced cooling of lattices.

Complex gene regulatory circuits contain many features that are likely to contribute to their operation. It is unclear, however, whether all these features are necessary for proper circuit behavior or whether certain ones are refinements that make the circuit work better but are dispensable for qualitatively normal behavior. We have addressed this question using the phage λ regulatory circuit, which can persist in two stable states, the lytic state and the lysogenic state. In the lysogenic state, the CI repressor positively regulates its own expression by stimulating transcription from the P RM promoter. We tested whether this feature is an essential part of the regulatory circuitry. Several phages with a c I mutation preventing positive autoregulation and an up mutation in the P RM promoter showed near-normal behavior. We conclude that positive autoregulation is not necessary for proper operation of the λ circuitry and speculate that it serves a partially redundant function of stabilizing a bistable circuit, a form of redundancy we term “circuit-level redundancy.” We discuss our findings in the context of a two-stage model for evolution and elaboration of regulatory circuits from simpler to more complex forms.