This enzyme catalyzes the hydrogen sulfide production from sulfite. It is strictly anaerobic. It is regulated by electron acceptors rather than by cysteine. 3 H2O + hydrogen sulfide + 3 NAD+ = 4 H+ + 3 NADH + sulfite.

MSIDIDIIKARAKNEYRLSKVRGEAMISVRIPGGILPAHLLTVARDIAETWGNGQIHLTT RQKLAMPGIRYEDIDNVNAALEPFLREIEIELCDVQVEDTKAGYLAIGGRNIVACQGNRI CQKANTDTTGLSRRLEKLVYPSPYHLKTVIVGCPNDCAKASMADLGIIGVAKMRFTADRC IGCGACVKACSHHAVGCLALKNGKAVKEESACIGCGECVLACPTLAWQRKPDQLWQVRLG GRTSKKTPRVGKLFLNWVTEDVIKQVIVNLYEFEKEMLGGKPIYLHMGHLIDKGGYLRFK ERVLRGVQLNPEAMVAERIYWAEDESVARMHLKPAGH342

	PMID	Title & Author	Abstract	Year
0	1704886	Sequence analysis and expression of the Salmonella typhimurium asr operon encoding production of hydrogen sulfide from sulfite.Huang CJ, Barrett EL	A chromosomal locus of Salmonella 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 asrC contained arrangements of cysteines characteristic of [4Fe-4S] 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. coli NADH-linked nitrite reductase. However, asr expression by the fusion plasmid was not affected by site-specific mutations within the apparent FNR-binding site.	1991
1	10231485	The genetic basis of tetrathionate respiration in Salmonella typhimurium.Hensel M, Hinsley AP, Nikolaus T, Sawers G, Berks BC	A range of bacteria are able to use tetrathionate as a terminal respiratory electron acceptor. Here we report the identification and characterization of the ttrRSBCA locus required for tetrathionate respiration in Salmonella typhimurium LT2a. The ttr genes are located within Salmonella pathogenicity island 2 at centisome 30.5. ttrA, ttrB and ttrC are the tetrathionate reductase structural genes. Sequence analysis suggests that TtrA contains a molybdopterin guanine dinucleotide cofactor and a [4Fe-4S] cluster, that TtrB binds four [4Fe-4S] clusters, and that TtrC is an integral membrane protein containing a quinol oxidation site. TtrA and TtrB are predicted to be anchored by TtrC to the periplasmic face of the cytoplasmic membrane implying a periplasmic site for tetrathionate reduction. It is inferred that the tetrathionate reductase, together with thiosulphate and polysulphide reductases, make up a previously unrecognized class of molybdopterin-dependent enzymes that carry out the reductive cleavage of sulphur-sulphur bonds. Cys-256 in TtrA is proposed to be the amino acid ligand to the molybdopterin cofactor. TtrS and TtrR are the sensor and response regulator components of a two-component regulatory system that is absolutely required for transcription of the ttrBCA operon. Expression of an active tetrathionate reduction system also requires the anoxia-responsive global transcriptional regulator Fnr. The ttrRSBCA gene cluster confers on Escherichia coli the ability to respire with tetrathionate as electron acceptor.	1999
2	15711167	Domain evolution and functional diversification of sulfite reductases	Sulfite reductases are key enzymes of assimilatory and dissimilatory sulfur metabolism, which occur in diverse bacterial and archaeal lineages. They share a highly conserved domain "C-X5-C-n-C-X3-C" for binding siroheme and iron-sulfur clusters that facilitate electron transfer to the substrate. For each sulfite reductase cluster, the siroheme-binding domain is positioned slightly differently at the N-terminus of dsrA and dsrB, while in the assimilatory proteins the siroheme domain is located at the C-terminus. Our sequence and phylogenetic analysis of the siroheme-binding domain shows that sulfite reductase sequences diverged from a common ancestor into four separate clusters (aSir, alSir, dsr, and asrC) that are biochemically distinct; each serves a different assimilatory or dissimilatory role in sulfur metabolism. The phylogenetic distribution and functional grouping in sulfite reductase clusters (dsrA and dsrB vs. aSiR, asrC, and alSir) suggest that their functional diversification during evolution may have preceded the bacterial/archaeal divergence.	2005

Huang C J , Barrett E L . Sequence analysis and expression of the Salmonella typhimurium asr operon encoding production of hydrogen sulfide from sulfite.[J]. Journal of Bacteriology, 1991, 173(4):1544-1553.