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In eukaryotic cells, intron lariats produced by the spliceosome contain a 2′5′ phosphodiester linkage. The RNA lariat debranching enzyme, Dbr1, is the only enzyme known to hydrolyze this bond. Dbr1 is a member of the metallophosphoesterase (MPE) family of enzymes, and recent X-ray crystal structures and biochemistry data demonstrate that Dbr1 from Entamoeba histolyticauses combinations of Mn2+, Zn2+, and Fe2+as enzymatic cofactors. Here, we examine the kinetic properties and metal dependence of the Dbr1 homolog from Saccharomyces cerevisiae(yDbr1). Elemental analysis measured stoichiometric quantities of Fe and Zn in yDbr1 purified following heterologous expression E. coli. We analyzed the ability of Fe2+, Zn2+, and Mn2+to reconstitute activity in metal-free apoenzyme. Purified yDbr1 was highly active, turning over substrate at 5.6 sec−1, and apo-yDbr1 reconstituted with Fe2+was the most active species, turning over at 9.2 sec−1. We treated human lymphoblastoid cells with the iron-chelator deferoxamine and measured a twofold increase in cellular lariats. These data suggest that Fe is an important biological cofactor for Dbr1 enzymes.

The RNA lariat debranching enzyme is the sole enzyme responsible for hydrolyzing the 2′-5′ phosphodiester bond in RNA lariats produced by the spliceosome. Here, we test the ability of Dbr1 to hydrolyze branched RNAs (bRNAs) that contain a 2′-5′-phosphorothioate linkage, a modification commonly used to resist degradation. We attempted to cocrystallize a phosphorothioate-branched RNA (PS-bRNA) with wild-type Entamoeba histolyticaDbr1 (EhDbr1) but observed in-crystal hydrolysis of the phosphorothioate bond. The crystal structure revealed EhDbr1 in a product-bound state, with the hydrolyzed 2′-5′ fragment of the PS-bRNA mimicking the binding mode of the native bRNA substrate. These findings suggest that product inhibition may contribute to the kinetic mechanism of Dbr1. We show that Dbr1 enzymes cleave phosphorothioate linkages at rates ∼10,000-fold more slowly than native phosphate linkages. This new product-bound crystal structure offers atomic details, which can aid inhibitor design. Dbr1 inhibitors could be therapeutic or investigative compounds for human diseases such as human immunodeficiency virus (HIV), amyotrophic lateral sclerosis (ALS), cancer, and viral encephalitis.

ABSTRACTIntrons 2 and 4 of the psbAgene of Chlamydomonas reinhardtiichloroplasts (Cr.psbA2andCr.psbA4, respectively) contain large free-standing open reading frames (ORFs). We used transformation of an intronless-psbAstrain (IL) to test whether these introns undergo homing. Each intron, plus short exon sequences, was cloned into a chloroplast expression vector in both orientations and then cotransformed into IL along with a spectinomycin resistance marker (16Srrn). For Cr.psbA2, the sense construct gave nearly 100% cointegration of the intron whereas the antisense construct gave 0%, consistent with homing. For Cr.psbA4, however, both orientations produced highly efficient cointegration of the intron. Efficient cointegration of Cr.psbA4also occurred when the intron was introduced as a restriction fragment lacking any known promoter. Deletion of most of the ORF, however, abolished cointegration of the intron, consistent with homing. TheCr.psbA4constructs also contained a 3-(3,4-dichlorophenyl)-1,1-dimethylurea resistance marker in exon 5, which was always present when the intron integrated, thus demonstrating exon coconversion. Remarkably, primary selection for this marker gave >100-fold more transformants (>10,000/μg of DNA) than did the spectinomycin resistance marker. A transhoming assay was developed for Cr.psbA4; the ORF-minus intron integrated when the ORF was cotransformed on a separate plasmid. This assay was used to identify an intronic region between bp −88 and −194 (relative to the ORF) that stimulated homing and contained a possible bacterial (−10, −35)-type promoter. Primer extension analysis detected a transcript that could originate from this promoter. Thus, this mobile, self-splicing intron also contains its own promoter for ORF expression. The implications of these results for horizontal intron transfer and organelle transformation are discussed.

Introns 2 and 4 of the psbAgene of Chlamydomonas reinhardtiichloroplasts (Cr.psbA2andCr.psbA4, respectively) contain large free-standing open reading frames (ORFs). We used transformation of an intronless-psbAstrain (IL) to test whether these introns undergo homing. Each intron, plus short exon sequences, was cloned into a chloroplast expression vector in both orientations and then cotransformed into IL along with a spectinomycin resistance marker (16Srrn). For Cr.psbA2, the sense construct gave nearly 100% cointegration of the intron whereas the antisense construct gave 0%, consistent with homing. For Cr.psbA4, however, both orientations produced highly efficient cointegration of the intron. Efficient cointegration of Cr.psbA4also occurred when the intron was introduced as a restriction fragment lacking any known promoter. Deletion of most of the ORF, however, abolished cointegration of the intron, consistent with homing. TheCr.psbA4constructs also contained a 3-(3,4-dichlorophenyl)-1,1-dimethylurea resistance marker in exon 5, which was always present when the intron integrated, thus demonstrating exon coconversion. Remarkably, primary selection for this marker gave >100-fold more transformants (>10,000/μg of DNA) than did the spectinomycin resistance marker. A transhoming assay was developed for Cr.psbA4; the ORF-minus intron integrated when the ORF was cotransformed on a separate plasmid. This assay was used to identify an intronic region between bp −88 and −194 (relative to the ORF) that stimulated homing and contained a possible bacterial (−10, −35)-type promoter. Primer extension analysis detected a transcript that could originate from this promoter. Thus, this mobile, self-splicing intron also contains its own promoter for ORF expression. The implications of these results for horizontal intron transfer and organelle transformation are discussed.

Abstract: The sequences and predicted secondary structures of the four catalytic group-I introns in the psbA gene of Chlamydomonas reinhardtii, Cr.psbA-1–Cr.psbA-4, have been determined. Cr.psbA-1 and Cr.psbA-4 are subgroup-IA1 introns and have similar secondary structures, except at the 3′ end where Cr.psbA-1 contains a large inverted-repeat domain. Cr.psbA-4 is closely related to intron 1 of the Chlamydomonas moewusii psbA gene, with which it shares the same location, high nucleotide identity in the core, and an identically placed ORF that shows 58% amino-acid identity. Cr.psbA-2 is a subgroup-IA3 intron, and shows similarities to the Chlamydomonas eugametos rRNA intron, Ce.LSU-1. Cr.psbA-3 is a subgroup-IA2 intron, and is remarkably similar to the T4 phage intron, sunY. Interestingly, a degenerate version of Cr.psbA-3 is located in the intergenic region between the chloroplast petA and petD genes. All four introns contain ORFs, which potentially code for basic proteins of 11–38 kDa. The ORFs in introns 2 and 3 contain variants of the GIY-YIG motif; however, the Cr.psbA-2 ORF is free-standing, whereas the Cr.psbA-3 ORF is contiguous and in-frame with the upstream exon. The Cr.psbA-4 ORF contains an H-N-H motif, and possibly a GIY-YIG motif. These data indicate that the C. reinhardtiipsbA introns have multiple origins, and illustrate some of the evolutionary DNA dynamics associated with group-I introns in Chlamydomonas.

No abstract available for this article.

Three major types of organizational change are identified (alterations in behavior, technology, and structure), and the respective impact of each is explored. The difficulties in assessing the significance of a change idea are noted, and some criteria are suggested by which such an assessment can be made (eg., the number of organizational elements affected by the change). The paper concludes by noting practice implications stemming from an understanding of the types of change and their significance for those participants who have limited formal power.

In previous research on helping the responses of subjects who received no information about a stranger's opinions and who received information confirming the other's similarity, expressed equally positive sentiment toward the stranger and helped equally often (and more than a third group who expressed negative sentiment after learning that the stranger held dis similar opinions). The present study, stimttlated by this anomaly, found that although the two groups expressed equally positive sentiment toward the other, they helped equally as often only when there was little temptation to do otherwise. With greater temptation the difference in the frequency of helping was comparatively large. This was interpreted as suggesting that information about another can both create and destroy promotive social bonds.

(1) The kinetic properties of single sodium channels modified by the pyrethroid fenvalerate have been analyzed by patch clamp techniques using the cultured mouse neuroblastoma cells. (2) Fenvalerate drastically prolonged the open time of single sodium channels from the normal value of 5 ms to several hundred milliseconds during a depolarizing pulse. The channels remained open after termination of a depolarizing pulse for as long as several seconds. (3) The channel lifetime varied with the membrane potential, attained a maximum at -70 mV, and decreased with hyperpolarization and depolarization from -70 mV. (4) Prolonged openings of the modified channels allowed a current-voltage curve for a single channel to be plotted by sweeping a ramp pulse. The single channel conductance had a value of 11 pS and was linear over potentials ranging from 0 to -100 mV. (5) Power density spectral analysis of the open channel current noise indicated a single Lorentzian curve with a cut-off frequency at 90 Hz, indicating that the increase in noise during channel opening resulted from a relatively slow kinetic process. (6) The probability of the channel being modified by fenvalerate was independent of the length of time during which the channel was opened. This observation suggests that channel modification had taken place before the channel opened. This study of the prolonged opening at the single channel level provides a new insight into open channel properties and the kinetics of channel modification.

No abstract available for this article.

Two field experiments investigate how information about aspects of the social environment affects willingness to help others who are similar or dissimilar. Subjects were pedestrians who found a lost packet of materials containing information about the opinions of the packet's loser and a survey report which claimed that the quality of life in the neighborhood had either improved or deteriorated. In the fiist study, the opinions were varied in order to create different degrees of opinion similarity between the loser and subject. Results showed that relatively slight differences between the loser's and subject's opinions made subjects unwilling to help (return the lost packet) when information about the social environment suggested a negative state of affairs, e.g., quality of life deteriorating. However, when information about the social environment suggested a positive state of affairs, subjects were willing to help, even if there was little similarity of opinion. In the second study, subjects learned that the loser of the packet agreed with their views on one opinion but disagreed on another. In this study, one of the two opinions concerned the same social issue as the one described in the survey report. Results demonstrated that subjects helped when there was agreement on the social issue that was also highlighted in survey reports of a negative state of affairs in the social environment and withheld help when there was disagreement on this issue. There were trends for the same result with information about a positive state of affairs in the social world.

Copper–zinc superoxide dismutase (SOD1) is a major antioxidant metalloenzyme that protects cells from oxidative damage by superoxide anions (O2–). Structural, biophysical, and other characteristics have in the past been compiled for mammalian SOD1s and for the highly homologous fungal and bovine SOD1s. Here, we characterize the biophysical properties of a plant SOD1 from tomato chloroplasts and present several of its crystal structures. The most unusual of these structures is a structure at low pH in which tSOD1 harbors zinc in the copper-binding site but contains no metal in the zinc-binding site. The side chain of D83, normally a zinc ligand, adopts an alternate rotameric conformation to form an unusual bidentate hydrogen bond with the side chain of D124, precluding metal binding in the zinc-binding site. This alternate conformation of D83 appears to be responsible for the previously observed pH-dependent loss of zinc from the zinc-binding site of SOD1. Titrations of cobalt into apo tSOD1 at a similar pH support the lack of an intact zinc-binding site. Further characterization of tSOD1 reveals that it is a weaker dimer relative to human SOD1 and that it can be activated in vivothrough a copper chaperone for the SOD1-independent mechanism.

Rats, reinforced for spacing their responses 20 seconds apart, used water-drinking as a means of "pacing" their response rate. "Timing" may be based on mediating response sequences such as drinking.

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