Sequence analysis and expression of the Salmonella typhimurium asr operon encoding production of hydrogen sulfide from sulfite

Abstract

A chromosomal locus of SalmoneUa typhimurium which complements S. typhimurium asr (anaerobic sulfite reduction) mutants and confers on Escherichia coli the ability to produce hydrogen sulfide from sulfite was recently cloned (C. J. Huang and E. L. Barrett, J. Bacteriol. 172:4100-4102, 1990). The DNA sequence and the transcription start site have been determined. Analysis of the sequence and gene products revealed a functional operon containing three genes which have been designated asrA, asrB, and asrC, encoding peptides of 40, 31, and 37 kDa, respectively. The predicted amino acid sequences of both asrA and asiC contained arrangements of cysteines characteristic of ferredoxins. The sequence of asrB contained a typical nucleotide-binding region. The sequence of asrC contained, in addition to the ferredoxinlike cysteine clusters, two other cysteine clusters closely resembling the proposed siroheme-binding site in biosynthetic sulfite reductase. Expression of lacZ fused to the asr promoter was repressed by oxygen and induced by sulfite.Analysis of promoter deletions revealed a region specific for sulfite regulation and a second region required for anaerobic expression. Computer-assisted DNA sequence analysis revealed a site just upstream of the first open reading frame which had significant homology to the FNR protein-binding site of E. coi NADH-linked nitrite reductase. However, asr expression by the fusion plasmid was not affected by site-specific mutations within the apparent FNR-binding site.The reduction of sulfite to sulfide by electrons from hydrogen or an organic substrate constitutes the central energy-conserving step in the metabolism of the sulfatereducing bacteria (37). Outside this group, dissimilatory sulfite reduction is rare. However, many microorganisms, including members of the Enterobacteriaceae family, assimilate sulfate by means of a cysteine biosynthetic pathway in which sulfite reduction is a step (30). The biosynthetic sulfite reductase in Escherichia coli and Salmonella typhimurium consists of a flavoprotein, encoded by cysJ, and a hemoprotein, encoded by cysI (30,38,40,51). In both the dissimilatory pathway of the sulfate reducers and the assimilatory pathway of the Enterobacteriaceae, siroheme and ironsulfur clusters are required participants (36,40). Siroheme biosynthesis is encoded by the cysG gene in E. coli and S. typhimurium (27). Mutations in cysI, cysJ, or cysG all result in cysteine auxotrophy.S. typhimurium differs from E. coli in two aspects of sulfite reduction. Firstly, it produces significant quantities of free hydrogen sulfide from sulfite, a property which, among the Enterobacteriaceae, is unique to the genera Salmonella and Edwardsiella (2, 41). Our studies of H2S production from sulfite by S. typhimurium have shown that it is strictly anaerobic (21), genetically distinct from its biosynthetic analog (21, 24), linked to NADH rather than NADPH oxidation (21), and regulated by available electron acceptors rather than by cysteine (6, 21, 24). Thus, it is essentially a dissi...

A New Type of Sulfite Reductase, a Novel Coenzyme F420-dependent Enzyme, from the Methanarchaeon Methanocaldococcus jannaschii

“…7B); a close relationship was also seen with the subunits of dissimilatory sulfite reductases, DsrA and DsrB. AsrC is a dissimilatory enzyme (39,40); the abbreviation Asr means anaerobic sulfite reduction and not assimilatory sulfite reductase (39). Therefore, Fsr-C belonged to the dissimilatory sulfite reductase group (AsrC, DsrA, and DsrB).…”

“…or DSR, and the siroheme containing subunit (AsrC) of a small sulfite reductase that is expressed in Salmonella enterica under anaerobic growth conditions (Fig. 5B) (33,(37)(38)(39). As explained in the following section, S. enterica AsrC is a dissimilatory sulfite reductase (DSR).…”

Characterization of the locus encoding the [Ni-Fe] sulfhydrogenase from the archaeon Pyrococcus furiosus: evidence for a relationship to bacterial sulfite reductases

“…In this communication we give the complete nucleotide sequence of the hydrogenase gene cluster. We have found that while two of the four subunits (a and S) are homologous to the large and the small subunits of dimeric and multimeric [Ni-Fe] hydrogenases, the p and y subunits share extensive homologies with the products of the sulfite reductase genes (asrA and asrB) from Salmonella tJphimt/ritrm (Huang & Barrett, 1991). These findings and the genetic relationship of the P. ftrrioszis byd genes to the other [Ni-Fe] hydrogenases are discussed in light of the biochemical properties of the P. ftlriosus enzyme.…”

Taurocholic acid metabolism by gut microbes and colon cancer

“…dsrAB type dissimilatory (bi)sulfite reductases are highly conserved and abundant in many bacteria and archaea due to lateral gene transfer. 58 The enzyme has been characterized in Salmonella enterica serovar Typhimurium 59 and Clostridium pasteurianum; 60 however, the majority of bacteria with this activity in humans are found in the Desulfovibrionaceae, a subdivision of the d-Proteobacteria. 61 Figure 1.…”