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We offer evidence for a signal that synchronizes the behavior of hundreds of Myxococcus xanthus cells in a growing swarm. Swarms are driven to expand by the periodic reversing of direction by members. By using time-lapse photomicroscopy, two organized multicellular elements of the swarm were analyzed: single-layered, rectangular rafts and round, multilayered mounds. Rafts of hundreds of cells with their long axes aligned in parallel enlarge as individual cells from the neighborhood join them from either side. Rafts can also add a second layer piece by piece. By repeating layer additions to a raft and rounding each layer, a regular multilayered mound can be formed. About an hour after a five-layered mound had formed, all of the cells from its top layer descended to the periphery of the fourth layer, both rapidly and synchronously. Following the first synchronized descent and spaced at constant time intervals, a new fifth layer was (re)constructed from fourth-layer cells, in very close proximity to its old position and with a number of cells similar to that before the "explosive" descent. This unexpected series of changes in mound structure can be explained by the spread of a signal that synchronizes the reversals of large groups of individual cells. [ABSTRACT FROM AUTHOR]

Weather and climate extremes have been varying and changing on many different time scales. In recent decades, heat waves have generally become more frequent across the United States, while cold waves have been decreasing. While this is in keeping with expectations in a warming climate, it turns out that decadal variations in the number of U.S. heat and cold waves do not correlate well with the observed U.S. warming during the last century. Annual peak flow data reveal that river flooding trends on the century scale do not show uniform changes across the country. While flood magnitudes in the Southwest have been decreasing, flood magnitudes in the Northeast and north-central United States have been increasing. Confounding the analysis of trends in river flooding is multiyear and even multidecadal variability likely caused by both large-scale atmospheric circulation changes and basin-scale "memory" in the form of soil moisture. Droughts also have long-term trends as well as multiyear and decadal variability. Instrumental data indicate that the Dust Bowl of the 1930s and the drought in the 1950s were the most significant twentieth-century droughts in the United States, while tree ring data indicate that the megadroughts over the twelfth century exceeded anything in the twentieth century in both spatial extent and duration. The state of knowledge of the factors that cause heat waves, cold waves, floods, and drought to change is fairly good with heat waves being the best understood. [ABSTRACT FROM AUTHOR]

The formation of spore-filled fruiting bodies by myxobacteria is a fascinating case of multicellular self-organization by bacteria. The organization of Myxococcus xanthus into fruiting bodies has long been studied not only as an important example of collective motion of bacteria, but also as a simplified model for developmental morphogenesis. Sporulation within the nascent fruiting body requires signaling between moving cells in order that the rod-shaped self-propelled cells differentiate into spores at the appropriate time. Probing the three-dimensional structure of myxobacteria fruiting bodies has previously presented a challenge due to limitations of different imaging methods. A new technique using Infrared Optical Coherence Tomography (OCT) revealed previously unknown details of the internal structure of M. xanthus fruiting bodies consisting of interconnected pockets of relative high and low spore density regions. To make sense of the experimentally observed structure, modeling and computer simulations were used to test a hypothesized mechanism that could produce high-density pockets of spores. The mechanism consists of self-propelled cells aligning with each other and signaling by end-to-end contact to coordinate the process of differentiation resulting in a pattern of clusters observed in the experiment. The integration of novel OCT experimental techniques with computational simulations can provide new insight into the mechanisms that can give rise to the pattern formation seen in other biological systems such as dictyostelids, social amoeba known to form multicellular aggregates observed as slugs under starvation conditions. [ABSTRACT FROM AUTHOR]

Using a recently homogenized observational daily maximum ( TMAX) and minimum temperature ( TMIN) dataset for China, the extreme temperatures from the 40-yr ECMWF Re-Analysis (ERA-40), the Japanese 25-year Reanalysis (JRA-25), the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2), and the ECMWF''s ERA-Interim (ERAIn) reanalyses for summer (June--August) and winter (December--February) are assessed by probability density functions for the periods 1979--2001 and 1990--2001. For 1979--2001, no single reanalysis appears to be consistently accurate across eight areas examined over China. The ERA-40 and JRA-25 reanalyses show similar representations and close skill scores over most of the regions of China for both seasons. NCEP-2 generally has lower skill scores, especially over regions with complex topography. The regional and seasonal differences identified are commonly associated with different geographical locations and the methods used to diagnose these quantities. All the selected reanalysis products exhibit better performance for winter compared to summer over most regions of China. The TMAX values from the reanalysis tend to be systematically underestimated, while TMIN is systematically closer to observed values than TMAX. Comparisons of the reanalyses to reproduce the 99.7 percentiles for TMAX and 0.3 percentiles for TMIN show that most reanalyses tend to underestimate the 99.7 percentiles in maximum temperature both in summer and winter. For the 0.3 percentiles in TMIN, NCEP-2 is relatively inaccurate with a −−12°°C cold bias over the Qinghai--Tibetan Plateau in winter. ERA-40 and JRA-25 generally overestimate the extreme TMIN, and the extreme percentage differences of ERA-40 and JRA-25 are quite similar over all of the regions. The results are generally similar for 1990--2001, but in contrast to the other three reanalysis products the newly released ERAIn is very reasonable, especially for wintertime TMIN, with a skill score greater than 0.83 for each region of China. This demonstrates the great potential of this product for use in future impact assessments on continental scales where those impacts are based on extreme temperatures. [ABSTRACT FROM AUTHOR]

In this paper, a novel framework is presented to estimate protein-protein interactions (PPIs) and domain-domain interactions (DDIs) based on a belief propagation estimation method that efficiently computes interaction probabilities. Experimental interactions, domain architecture, and gene ontology (GO) annotations are used to create a factor graph representation of the joint probability distribution of pairwise protein and domain interactions. Bound structures are used as a priori evidence of domain interactions. These structures come from experiments documented in iPfam. The probability distribution contained in the factor graph is then efficiently marginalized with a message passing algorithm called the sum-product algorithm (SPA). This method is compared against two other approaches: maximum- likelihood estimation (MLE) and maximum specificity set cover (MSSC). SPA performs better for simulated scenarios and for inferring high-quality PPI data of Saccharomyces cerevisiae. This framework can be used to predict potential protein and domain interactions at a genome wide scale and for any organism with identified protein-domain architectures. [ABSTRACT FROM AUTHOR]

Myxococcus xanthus creates complex and dynamic multicellular patterns as it swarms. The cells have two polar gliding engines: pulling type IV pili at their leading pole and pushing slime secretory nozzles at their lagging pole. Evidence is presented that slime secretion is vital for cell survival and that the peptidoglycan/cytoskeleton serves as a template to keep both engines oriented in the same direction. Swarming requires that each cell periodically reverse its gliding direction. For the leading pole to become the trailing pole, old engines are inactivated at both ends while new engines are being created at both ends. Reversal is initiated by a small G-protein reversal switch; a pulse of frzE∼P from a reversal clock triggers MglA to bind GTP. Mgl·GTP then recognizes the engines that are currently in use and inactivates both of them. Meanwhile, new engines appear as instructed by the template, and the cell starts to glide in the opposite direction. [ABSTRACT FROM AUTHOR]

Plant ecologists have long been concerned with a seemingly paradoxical scenario in the relationship between plant growth and climate change: warming may actually increase the risk of plant frost damage. The underlying hypothesis is that mild winters and warm, early springs, which are expected to occur as the climate warms, may induce premature plant development, resulting in exposure of vulnerable plant tissues and organs to subsequent late-season frosts. The 2007 spring freeze in the eastern United States provides an excellent opportunity to evaluate this hypothesis and assess its large-scale consequences. In this article, we contrast the rapid prefreeze phenological advancement caused by unusually warm conditions with the dramatic postfreeze setback, and report complicated patterns of freeze damage to plants. The widespread devastation of crops and natural vegetation occasioned by this event demonstrates the need to consider large fluctuations in spring temperatures a real threat to terrestrial ecosystem structure and functioning in a warming climate. [ABSTRACT FROM AUTHOR]

Swarming, a collective motion of many thousands of cells, produces colonies that rapidly spread over surfaces. In this paper, we introduce a cell-based model to study how interactions between neighboring cells facilitate swarming. We chose to study Myxococcus xanthus, a species of myxobacteria, because it swarms rapidly and has well-defined cell-cell interactions mediated by type IV pili and by slime trails. The aim of this paper is to test whether the cell contact interactions, which are inherent in pili-based S motility and slime-based A motility, are sufficient to explain the observed expansion of wild-type swarms. The simulations yield a constant rate of swarm expansion, which has been observed experimentally. Also, the model is able to quantify the contributions of S motility and A motility to swarming. Some pathogenic bacteria spread over infected tissue by swarming. The model described here may shed some light on their colonization process. [ABSTRACT FROM AUTHOR]

Myxococcus leaves a trail of slime on agar as it moves. A filament of slime can be seen attached to the end of a cell, but it is seen only at one end at any particular moment. To identify genes essential for A motility, transposon insertion mutations with defective A motility were studied. Fifteen of the 33 mutants had totally lost A motility. All these mutant cells had filaments of slime emerging from both ends, indicating that bipolar secretion prevents A motility. The remaining 18 A motility mutants, also produced by gene knockout, secreted slime only from one pole, but they swarmed at a lower rate than A+ and are called ‘partial’ gliding mutants, or pgl. For each pgl mutant, the reduction in swarm expansion rate was directly proportional to the reduction in the coefficient of elasticotaxis. The pgl mutants have a normal reversal frequency and normal gliding speed when they move. But their probability of movement per unit time is lower than pgl+ cells. Many of the pgl mutants are produced by transposon insertions in glycosyltransferase genes. It is proposed that these glycosyltransferases carry out the synthesis of a repeat unit polysaccharide that constitutes the slime. [ABSTRACT FROM AUTHOR]

Myxobacterial cells are social; they swarm by gliding on surfaces as they feed cooperatively. When they sense starvation, tens of thousands of cells change their movement pattern from outward spreading to inward concentration and form aggregates that become fruiting bodies. Cells inside fruiting bodies differentiate into round, nonmotile, environmentally resistant spores. Traditionally, cell aggregation has been considered to imply chemotaxis, a long-range cell interaction that shares many features of chemical reaction-diffusion dynamics. The biological evidence, however, suggests that Myxococcus xanthus aggregation is the consequence of direct cell-contact interactions that are different from chemotaxis. To test whether local interactions suffice to explain the formation of fruiting bodies and the differentiation of spores within them, we have simulated the process. In this article, we present a unified 3D model that reproduces in one continuous simulation all the stages of fruiting-body formation that have been experimentally observed: nonsymmetric initial aggregates (traffic jams), streams, formation of toroidal aggregates, hemispherical 3D mounds, and finally sporulation within the fruiting body. [ABSTRACT FROM AUTHOR]

Fortunately, I began research in 1950 when the basic concepts of microbial genetics could be explored experimentally. I began with bacteriophage λ and tried to establish the colinearity of its linkage map with its DNA molecule. My students and I worked out the regulation of λ repressor synthesis for the establishment and maintenance of lysogeny. We also investigated the proteins responsible for assembly of the phage head. Using cell extracts, we discovered how to package DNA inside the head in vitro. Around 1972, I began to use molecular genetics to understand the developmental biology of Myxococcus xanthus. In particular, I wanted to learn how myxococcus builds its multicellular fruiting body within which it differentiates spores. We identified two cell-to-cell signals used to coordinate development. We have elucidated, in part, the signal transduction pathway for C-signal that directs the morphogenesis of a fruiting body. [ABSTRACT FROM AUTHOR]

We analyzed several features of five currently available δ-proteobacterial genomes, including two aerobic bacteria exhibiting predatory behavior and three anaerobic sulfate-reducing bacteria. The δ genomes are distinguished from other bacteria by several properties: (i) The δ genomes contain two ‘giant’ S1 ribosomal protein genes in contrast to all other bacterial types, which encode a single or no S1; (ii) in most δ-proteobacterial genomes the major ribosomal protein (RP) gene cluster is near the replication terminus whereas most bacterial genomes place the major RP cluster near the origin of replication; (iii) the δ genomes possess the rare combination of discriminating asparaginyl and glutaminyl tRNA synthetase (AARS) together with the amidotransferase complex (Gat CAB) genes that modify Asp-tRNAAsn into Asn-tRNAAsn and Glu-tRNAGln into Gln-tRNAGln; (iv) the TonB receptors and ferric siderophore receptors that facilitate uptake and removal of complex metals are common among δ genomes; (v) the anaerobic δ genomes encode multiple copies of the anaerobic detoxification protein rubrerythrin that can neutralize hydrogen peroxide; and (vi) σ54 activators play a more important role in the δ genomes than in other bacteria. δ genomes have a plethora of enhancer binding proteins that respond to environmental and intracellular cues, often as part of two-component systems; (vii) δ genomes encode multiple copies of metallo-β-1actamase enzymes; (viii) a host of secretion proteins emphasizing SecA, SecB, and SecY may be especially useful in the predatory activities of Myxococcus xanthus; (ix) δ proteobacteria drive many multiprotein machines in their periplasms and outer membrane, including chaperone-feeding machines, jets for slime secretion, and type IV pill. Bdellovibrio replicates in the periplasm of prey cells. The sulfate-reducing δ proteobacteria metabolize hydrogen and generate a proton gradient by electron transport. The predicted highly expressed genes from δ genomes reflect their different ecologies, metabolic strategies, and adaptations. [ABSTRACT FROM AUTHOR]

The type IV pilus filament of Myxococcus xanthus penetrates the outer membrane through a gated channel – the PilQ secretin. Assembly of the channel and formation of PilQ multimeric complexes that resist disassembly in heated detergent is correlated with the release of a 50 kDa fragment of PilQ. Tgl lipoprotein is required for PilQ assembly in M. xanthus, because PilQ monomers but no heat and detergent-resistant complexes are present in a strain from which tgl has been deleted. PilQ protein is often found in single patches at both poles of the cell. Tgl, however, is found in a patch at only one pole that most likely identifies the piliated cell pole. Tgl protein that has been transferred from another cell by contact stimulation leads to secretin assembly in the recipient. Pilus proteins PilQ, PilG, PilM, PilN, PilO and PilP are also required for the donation of Tgl by contact stimulation to a stimulation recipient. We suggest that these proteins are parts of a polar superstructure that holds PilQ monomers in a cluster and ready for Tgl to bring about secretin assembly. [ABSTRACT FROM AUTHOR]

Myxobacteria provide one of the simplest models of cell-cell interaction and organized cell movement leading to cellular differentiation. When starved, tens of thousands of cells change their movement pattern from outward spreading to inward concentration; they form aggregates that become fruiting bodies. Cells inside fruiting bodies differentiate into round, nonmotile, environmentally resistant spores. Traditionally, cell aggregation has been considered to imply chemotaxis; a long-range cell interaction. However, myxobacterial aggregation is the consequence of direct cell-contact interactions, not chemotaxis. We present here a 3D stochastic lattice-gas cellular automata model of cell aggregation based on local cell-cell contact, and no chemotaxis. We demonstrate that a 3D discrete stochastic model can simulate two stages of cell aggregation. First, a "traffic jam" forms embedded in a field of motile cells. The jam then becomes an aggregation center that accumulates more cells. We show that, at high cell density, cells stream around the traffic jam, generating a 3D hemispherical mound. Later, when the nuclear traffic jam dissolves, the aggregation center becomes a 3D ring of streaming cells. [ABSTRACT FROM AUTHOR]

In response to starvation, Myxococcus xanthus initiates a developmental program that results in the formation of spore-filled, multicellular fruiting bodies. Many developmentally regulated genes in M. xanthus are transcribed from σ54 promoters, and these genes require enhancer-binding proteins. Here we report the finding of an unusual group of 12 genes encoding σ54-dependent enhancer-binding proteins containing a forkhead-associated (FHA) domain as their N-terminal sensory domain. FHA domains in other proteins recognize phosphothreonine residues. An insertion mutation in one of these genes, Mx4885, caused a cell autonomous aggregation and sporulation defect. In-frame deletion mutants showed that the FHA domain is necessary for proper Mx4885 function. The altered pattern of developmental gene expression in the mutant implied that Mx4885 is on the pathway of response to the morphogenetic C-signal. Immunoblots specific for C-signal and FruA imply that the site of Mx4885 action is downstream of FruA synthesis on the C-signal transduction pathway. Mx4885 may help to coordinate the level of intracellular phosphorylated FruA (FruA-P) with the level of C-signal displayed on the signal donor cell. Because FHA domains respond to phosphothreonine-containing proteins, these results suggest a regulatory link to the abundant Ser/Thr protein kinases in M. xanthus. [ABSTRACT FROM AUTHOR]

During development Myxococcus xanthus cells produce a series of spatial patterns by coordinating their motion through a contact-dependent signal, the C-signal. C-signaling modulates the frequency at which cells reverse their gliding direction. It does this by interacting with the Frz system (a homolog of the Escherichia coli chemosensory system) via a cascade of covalent modifications. Here we show that introducing a negative feedback into this cascade results in oscillatory behavior of the signaling circuit. The model explains several aspects of M. xanthus behavior during development including the nonrandom distribution of reversal times, and the differences in response of the reversal frequency to both moderate and high levels of C-signaling at different developmental stages. We also propose experiments to test the model. [ABSTRACT FROM AUTHOR]

Myxobacteria use soluble and cell-contact signals during their star-vation- induced formation of fruiting bodies. These signals coordinate developmental gene expression with the cell movements that build fruiting bodies. Early in development, the quorum-sensing A-signal in Myxococcus xanthus helps to assess starvation and induce the first stage of aggregation. Later, the morphogenetic C-signal helps to pattern cell movement and shape the fruiting body. C-signal is a 17-kDa cell surface protein that signals by contact between the ends of two cells. The number of C-signal molecules per cell rises 100-fold from the beginning of fruiting body development to the end, when spores are formed. Traveling waves, streams, and sporulation have increasing thresholds for C-signal activity, and this progression ensures that spores form inside fruiting bodies. [ABSTRACT FROM AUTHOR]

Type IV pili are an efficient and versatile device for bacterial surface motility. They are widespread among the β-, γ-, and δ-proteobacteria and the cyanobacteria. Within that diversity, there is a core of conserved proteins that includes the pilin (PilA), the motors PilB and PilT, and various components of pilus biogenesis and assembly, PilC, PilD, PilM, PilN, PilO, PilP, and PilQ. Progress has been made in understanding the motor and the secretory functions. PilT is a motor protein that catalyzes pilus retraction; PilB may play a similar role in pilus extension. Type IV pili are multifunctional complexes that can act as bacterial virulence factors because pilus-based motility is used to spread pathogens over the surface of a tissue, or to build multicellular structures such as biofilms and fruiting bodies. Copyright © 2004 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Under starvation conditions, a population of myxobacteria aggregates to build a fruiting body whose shape is species-specific and within which the cells sporulate. Early in this process, cells often pass through a "ripple phase" characterized by traveling linear, concentric, and spiral waves. These waves are different from the waves observed during slime mold aggregation that depend on diffusible morphogens, because myxobacteria communicate by direct contact. The difference is most dramatic when waves collide: rather than annihilating one another, myxobacterial waves appear to pass through one another unchanged. Under certain conditions, the spacing and location of the nascent fruiting bodies is determined by the wavelength and pattern of the waves. Later in fruiting body development, waves are replaced by streams of cells that circulate around small initial aggregates enlarging and rounding them. Still later, pairs of motile aggregates coalesce to form larger aggregates that develop into fruiting bodies. Here we present a mathematical model that quantitatively explains these wave and aggregation phenomena. [ABSTRACT FROM AUTHOR]

Develops a mathematical model to observe the pattern formation and traveling waves of cell in myxobacteria. Difference of the wave patterns from other observations in social organisms; Aggregation of the myxobacteria cell population under starvation conditions; Characteristics of waves observed in the ripple phase.

Investigates cell behavior in traveling wave patterns of myxobacteria. Contribution of cell movement to the wave pattern; Occurrence of cell movement along the same axis; Presence of a high degree of cell alignment parallel to the direction of rippling.

Multicellular organisms appear to have arisen from unicells numerous times. Multicellular cyanobacteria arose early in the history of life on Earth. Multicellular forms have since arisen independently in each of the kingdoms and several times in some phyla. If the step from unicellular to multicellular life was taken early and frequently, the selective advantage of multicellularity may be large. By comparing the properties of a multicellular organism with those of its putative unicellular ancestor, it may be possible to identify the selective force(s). The independent instances of multicellularity reviewed indicate that advantages in feeding and in dispersion are common. The capacity for signaling between cells accompanies the evolution of multicellularity with cell differentiation. [ABSTRACT FROM AUTHOR]

The C-signal is a morphogen that controls the assembly of fruiting bodies and the differentiation of myxospores. Production of this signal, which is encoded by the csgA gene, is regulated by the act operon of four genes that are co-transcribed from the same start site. The act A and act B genes regulate the maximum level of the C-signal, which never rises above one-quarter of the maximum wild-type level of CsgA protein in null mutants of either gene. The act A and act B mutants have the same developmental phenotype: both aggregate, neither sporulates, both prolong rippling. By sequence homology, act A encodes a response regulator, and act B encodes a sigma-54 activator protein of the NTRC class. The similar phenotypes of act A and act B deletion mutants suggest that the two gene products are part of the same signal transduction pathway. That pathway responds to C-signal and also regulates the production of CsgA protein, thus creating a positive feedback loop. The act C and act D genes regulate the time pattern of CsgA production, while achieving the same maximum level. An act C null mutant raises CsgA production 15 h earlier than the wild type, whereas an act D null mutant does so 6 h later than wild type. The loop explains how the C-signal rises continuously from early development to a peak at the time of sporulation, and the act genes govern the time course of that rise. [ABSTRACT FROM AUTHOR]

Presents information on a study which described the ability of Myxococcus xanthus cells to sense and respond to elastic forces within an agar gel on which they rest. Research design and methodology; Results and discussion.

Examines the mediation of cell motility by type IV pili (Tfp) without the use of flagella. Assessment on the social gliding in Myxococcus xanthus; Localization of Tfp; Formation of a coiled-coil in the signature N-terminal domain of Tfp.

Reports on the findings of a study which suggests that alignment enhances the cell-to-cell transfer of pilus phenotype. Details on the requirements of social gliding motility of Myxococcus xanthus; Suggestion that stimulation does not occur in liquid; Methodology used in the study.

Social gliding motility in Myxococcus xanthus depends on the presence of Type IV pili. To begin to examine the role of pili in social motility, 17 mutants were identified which had lost social motility, but still expressed pili. Four of these mutants carry point mutations which mapped to a locus upstream of the recently identified pilS, pilR, and pilA genes. Sequencing of this locus revealed a gene with homology to pilT from Pseudomonas aeruginosa. Sequencing of the four point mutations revealed that they occurred within the M. xanthus pilT locus. A markerless deletion within M. xanthus pilT, similar to the four point mutations, disrupted social gliding behaviour but did not interfere with pilus formation or pilus-dependent cell--cell agglutination. Using time-lapse videomicroscopy, residual social motility was observed in dsp- strains (known to be deficient in fibril but not pilus production); this was not observed in a ΔpilT dsp- double mutant. Two genes flanking pilT were also sequenced, and found to have homology to pilB and pilC from P. aeruginosa. Markerless deletions within these genes caused both pilus and social-motility defects. These results indicate that M. xanthus pilB and pilC are required for pilus biogenesis, while pilT is required for assembled pill to play their role in social motility. Thus, pilB, pilT, pilC, pilS, pilR and pilA form a contiguous cluster of pil genes required for social motility. [ABSTRACT FROM AUTHOR]

The social gliding behaviour of Myxococcus xanthus has previously been associated with the presence of polar pili. A Tn5 transposon insertion was isolated which introduces a defect in social gliding and is genetically linked to a known sgl locus; this insertion was found also to cause a piliation defect. A 2.7 kb section of DNA was isolated from either side of this transposon and sequenced, revealing three genes which encode amino acid sequences with substantial similarity to components of the Type IV pilus biogenesis pathway in Pseudomonas aeruginosa. The myxococcal pilA gene encodes a putative pilin precursor with a short signal sequence and processing site similar to those of other Type IV pilins. Myxococcal pilS and pilR encode amino acid sequences with similarity to PilS and PilR of P. aeruginosa, as well as to other members of the NtrB/C family of two-component regulators. Mutations within pilR and pilA that have no polar effect were demonstrated to be responsible for pilus and social motility defects. These results indicate that the pili of M. xanthus belong to the Type IV family of pili, and demonstrate that these pili are actually required for social motility. [ABSTRACT FROM AUTHOR]

Describes the cloning of the rpoN gene encoding the transcription factor sigma54 in Myxococcus xanthus. Existence of an ORF 1518 bp long that encodes a well-conserved member of the sigma54 family of sigma factors; Impossibility of constructing a viable null haploid under any of a wide range of growth conditions.

Discusses how myxobacteria use extracellular signals to sequentially check the density and arrangement of cells at different stages during development. Density-dependent growth; Control of cell movement; Fruiting body morphogenesis and development; A-signaling in early development; Identification of and response to A-factor; A-signal specificity; C-signal transmission and response.

Examines the effects of the pilH gene on ABC transporter homologue required for type IV pilus biogenesis and social gliding motility in Myxococcus xanthus. Lack of social motility of null mutants of pilG, pilH and pill; Deficiency in pilus production.