Mutation in the structural gene for release factor 1 (RF-1) of Salmonella typhimurium inhibits cell division

A Short-Term View of Protein Sequence Evolution from Salmonella

Much of the study of protein sequence evolution is based on sequence changes inferred to have occurred in nature. The sequences compared for this purpose are usually sufficiently distant that purifying selection has had nearly its full effect and most of the changes inferred have been exposed to a variety of conditions. Here, I make use of large numbers of Salmonella genome sequences to study changes known to be of very recent origin because they are inferred from comparison of very closely related sequences. The effects of purifying selection are weak yet discernible on this short timescale: the ratio of nonsynonymous to synonymous changes is smaller than expected under selective neutrality, but only slightly so. Essential genes have lower rates of nonsynonymous change, as they do on a longer timescale, but much more of this association remains after controlling for expression level. Positive selection for nonsynonymous change is inferred for 151 genes. For nearly half of these, this is attributable to selection for loss of function. Other forms of positive selection inferred include selection for amino acid changes that make enzymes less sensitive to antibiotics and selection for activating changes to proteins involved in transcriptional regulation. Positively selected variants of many genes are likely favored only under unusual conditions and disfavored in the long term, making detection of the positive selection with more distant comparisons difficult or impossible. The short-term view provided by close comparisons complements the long-term view obtained from more distant comparisons such as those between species.

Identification of a protein subset of the anthrax spore immunome in humans immunized with the anthrax vaccine adsorbed preparation

We identified spore targets of Anthrax Vaccine Adsorbed (AVA)-induced immunity in humans by screening recombinant clones of a previously generated, limited genomic Bacillus anthracis Sterne (pXO1(+), pXO2(-)) expression library of putative spore surface (spore-associated [SA]) proteins with pooled sera from human adults immunized with AVA (immune sera), the anthrax vaccine currently approved for use by humans in the United States. We identified 69 clones that reacted specifically with pooled immune sera but not with pooled sera obtained from the same individuals prior to immunization. Positive clones expressed proteins previously identified as localized on the anthrax spore surface, proteins highly expressed during spore germination, orthologs of proteins of diverse pathogens under investigation as drug targets, and orthologs of proteins contributing to the virulence of both gram-positive and gram-negative pathogens. Among the reactive clones identified by this immunological screen was one expressing a 15.2-kDa hypothetical protein encoded by a gene with no significant homology to sequences contained in databases. Further studies are required to define the subset of SA proteins identified in this study that contribute to the virulence of this pathogen. We hypothesize that optimal delivery of a subset of SA proteins identified by such studies to the immune system in combination with protective antigen (PA), the principal immunogen in AVA, might facilitate the development of defined, nonreactogenic, more-efficacious PA-based anthrax vaccines. Future studies might also facilitate the identification of SA proteins with potential to serve as targets for drug design, spore inactivation, or spore detection strategies.

A Salmonella enterica serovar typhimurium hemA mutant is highly susceptible to oxidative DNA damage

The first committed step in the biosynthesis of heme, an important cofactor of two catalases and a number of cytochromes, is catalyzed by the hemA gene product. Salmonella enterica serovar Typhimurium hemA26::Tn10d (hemA26) was identified in a genetic screen of insertion mutants that were sensitive to hydrogen peroxide. Here we show that the hemA26 mutant respires at half the rate of wild-type cells and is highly susceptible to the effects of oxygen species. Exposure of the hemA26 strain to hydrogen peroxide results in extensive DNA damage and cell death. The chelation of intracellular free iron fully abrogates the sensitivity of this mutant, indicating that the DNA damage results from the iron-catalyzed formation of hydroxyl radicals. The inactivation of heme synthesis does not change the amount of intracellular iron, but by diminishing the rate of respiration, it apparently increases the amount of reducing equivalents available to drive the Fenton reaction. We also report that hydrogen peroxide has opposite effects on the expression of hemA and hemH, the first and last genes of heme biosynthesis pathway, respectively. hemA mRNA levels decrease, while the transcription of hemH is induced by hydrogen peroxide, in an oxyR-dependent manner. The oxyR-dependent induction is suppressed under conditions that accelerate the Fenton reaction by a mechanism that is not yet understood.

The tubulin ancestor, FtsZ, draughtsman, designer and driving force for bacterial cytokinesis

We discuss in this review the regulation of synthesis and action of FtsZ, its structure in relation to tubulin and microtubules, and the mechanism of polymerization and disassembly (contraction) of FtsZ rings from a specific nucleation site (NS) at mid cell. These topics are considered in the light of recent immunocytological studies, high resolution structures of some division proteins and results indicating how bacteria may measure their mid cell point.