The Salmonella enterica serovar Typhimurium znuABC genes encoding a high-affinity zinc uptake system and its regulatory zur gene have been cloned. Salmonella serovar Typhimurium zur and znuC knockout mutants have been constructed by marker exchange. The 50% lethal dose of the znuC mutant increased when either orally or intraperitoneally inoculated in BALB/c mice, while virulence of the zur mutant decreased only when mice were intraperitoneally challenged.

Schizosaccharomyces pombe triggers different signalling pathways depending on the severity of the oxidative stress exerted, the main ones being the Pap1 and the Sty1 pathways. The Pap1 transcription factor is more sensitive to hydrogen peroxide (H(2)O(2)) than the MAP kinase Sty1 pathway, and is designed to induce adaptation, rather than survival, responses. The peroxiredoxin Tpx1 acts as a H(2)O(2) sensor and the upstream activator of the Pap1 pathway. Therefore, sensitivity to H(2)O(2) depends on this thioredoxin peroxidase. In order to achieve maximal activation of the MAP kinase pathway, the concentration of H(2)O(2) needs to be at least fivefold higher than that to fully activate Pap1. Tpx1 is a H(2)O(2) scavenger, thus its peroxidase activity is essential for aerobic growth. As described for other eukaryotic peroxiredoxins, high doses of H(2)O(2) temporarily inactivate Tpx1 and delay Pap1 activation, whereas the Sty1 pathway remains fully functional under these conditions. As part of the Sty1-dependent transcriptional response, the expression of Srx1 is induced and this reductase re-activates the over-oxidised Tpx1. Therefore, the antioxidant pathways of the fission yeast are perfectly designed so that the transcriptional programs triggered by the different signalling pathways never overlap.

Peroxiredoxins are known to interact with hydrogen peroxide (H 2 O 2 ) and to participate in oxidant scavenging, redox signal transduction, and heat-shock responses. The two-cysteine peroxiredoxin Tpx1 of Schizosaccharomyces pombe has been characterized as the H 2 O 2 sensor that transduces the redox signal to the transcription factor Pap1. Here, we show that Tpx1 is essential for aerobic, but not anaerobic, growth. We demonstrate that Tpx1 has an exquisite sensitivity for its substrate, which explains its participation in maintaining low steady-state levels of H 2 O 2 . We also show in vitro and in vivo that inactivation of Tpx1 by oxidation of its catalytic cysteine to a sulfinic acid is always preceded by a sulfinic acid form in a covalently linked dimer, which may be important for understanding the kinetics of Tpx1 inactivation. Furthermore, we provide evidence that a strain expressing Tpx1.C169S, lacking the resolving cysteine, can sustain aerobic growth, and we show that small reductants can modulate the activity of the mutant protein in vitro, probably by supplying a thiol group to substitute for cysteine 169. INTRODUCTIONPeroxiredoxins (Prxs) are a family of antioxidant enzymes that reduce hydrogen peroxide (H 2 O 2 ) and/or alkyl hydroperoxides to yield water and/or alcohol, using reducing equivalents provided principally by thioredoxin. These H 2 O 2 scavengers have been isolated from all kingdoms (Chae et al., 1994b), with six mammalian isoforms distributed in different organelles (Rhee et al., 2005b). Three types of structurally different Prxs have been described: 1-cysteine (Cys), 2-Cys, and atypical 2-Cys (for review, see Wood et al., 2003b). All three types of Prxs are dimers in solution, and they all share the same catalytic mechanism, in which the peroxidatic Cys, located close to the N-terminal domain, is oxidized to a sulfenic acid by either H 2 O 2 or alkyl hydroperoxides. In both types of 2-Cys Prxs, the sulfenic acid then reacts with the C-terminal (or resolving) Cys of the other subunit to form an intermolecular disulfide (classical 2-Cys Prxs), or with the C-terminal Cys of the same monomer to form an intramolecular disulfide (atypical 2-Cys Prxs). In both cases, the disulfides are specifically reduced by the thioredoxin and thioredoxin reductase system, with the exception of some prokaryotic Prxs, such as the Escherichia coli AhpC, which uses another protein, AhpF, for the regeneration of the reduced Prx. In the 1-Cys Prxs, the sulfenic acid is directly reduced to thiol, because there is no nearby Cys available to form a disulfide bond; the source of the reducing equivalents for regenerating this thiol is not known, although glutathione (GSH) has been proposed to serve as the electron donor in this reaction (Kang et al., 1998b).Prx activity can be regulated by phosphorylation (Chang et al., 2002), and possibly by changes in oligomerization states (Wood et al., 2002(Wood et al., , 2003b. Furthermore, some Prxs suffer an oxidation of one of their Cys residues, the peroxidatic Cys, to sulfi...

SummaryThe SOS response comprises a set of cellular functions aimed at preserving bacterial cell viability in front of DNA injuries. The SOS network, negatively regulated by the LexA protein, is found in many bacterial species that have not suffered major reductions in their gene contents, but presents distinctly divergent LexA-binding sites across the Bacteria domain. In this article, we report the identification and characterization of an imported multiple gene cassette in the Gamma Proteobacterium Pseudomonas putida that encodes a LexA protein, an inhibitor of cell division (SulA), an error-prone polymerase (DinP) and the alpha subunit of DNA polymerase III (DnaE). We also demonstrate that these genes constitute a DNA damage-inducible operon that is regulated by its own encoded LexA protein, and we establish that the latter is a direct derivative of the Gram-positive LexA protein. In addition, in silico analyses reveal that this multiple gene cassette is also present in many Proteobacteria families, and that both its gene content and LexA-binding sequence have evolved over time, ultimately giving rise to the lexA lineage of extant Gamma Proteobacteria.

The LexA regulon encompasses an ensemble of genes involved in preserving cell viability under massive DNA damage and is present in most bacterial phyla. Up to date, however, the scope of this network had only been assessed in the Gamma Proteobacteria. Here, we report the structure of the LexA regulon in the Alpha Proteobacteria, using a combined approach that makes use of in vitro and in vivo techniques to assist and validate the comparative genomics in silico methodology. This leads to the first experimentally validated description of the LexA regulon in the Alpha Proteobacteria, and comparison of regulon core structures in both classes suggests that a least common multiple set of genes (recA, ssb, uvrA and ruvCAB) might be a defining property of the Proteobacteria LexA network.