Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 253

Adjacent to the genes for dissimilatory sulfite reduction in D. vulgaris Hisa
gene that encodes for DsvD, a peptide of only 78 amino acids [ 108 ]. A similar gene
encoding 77 amino acids is also present in the genome of Ar. fulgidus and it is
downstream of the dsrB gene. The DsvD, also known as DsrD, is not the

subunit
and would not be associated with electron transfer because it lacks cysteine residues
[ 97 ]. DsvD has structural homology to DNA-binding proteins and may have a role
in transcription or translation [ 109 ].
D. gigas is a unique sulfate reducer in the sense that it has a multimeric
dissimilatory sulfite reductase. Dsr-I and Dsr-II have enzymatic activity while
Dsr-III is inactive [ 92 ]. The ʱ 2 ʲ 2 ʳ 2 structure of Dsr-I type from D. gigas contains
eight [4Fe-4S] clusters, two planar sirohydrochlorins and two saddle-shaped
sirohemes while the Dsr-II type contains two sirohemes, two hydrochlorins, two
[3Fe-4S] clusters, and six [4Fe-4S] clusters [ 92 ]. Dsr-III from D. gigas has iron-
sulfur clusters similar to Dsr-II and the inactivity of Dsr-III is attributed to the
absence of iron in the siroheme [ 92 ].
Analysis of the sirohydrochlorin from several Desulfovibrio species reveals that
it is different from that isolated from Escherichia ( E. ) coli. Notably, one of the eight
carboxylates of the tetrapyrrole moiety is replaced by an amide group at the
2'-acetate [ 110 ]. Additional [4Fe-4S] clusters, referred to as remote iron-sulfur
clusters, are located on the surfaces of the

ʳ

subunits. In D. gigas , the S atom
of sulfite binds to the Fe of the siroheme and a positively charged arginine along
with two lysine moieties in the

ʱ and ʲ

subunit has an electrostatic interaction with the
three oxygen atoms on sulfite. Another arginine forms hydrogen bonding with
sulfite and directs the release of sulfide and water molecules through a positively
charged water channel.
The

ʱ

ʳ subunit of D. gigas Dsr-I is aligned adjacent to the ʱ / ʲ subunits and the

C-terminus of the subunit is proposed to have multiple conformations that may

influence catalytic function. It has been proposed that activity of the C-terminus of
the

ʳ

subunit could explain the mechanism for three products (sulfide, thiosulfate,
and trithionate) produced by Dsr [ 92 ]. The association of dsr genes in D. gigas and
D. vulgaris H is given in Figure 4 .

ʳ

2.4.1.2 P-582-Type Sulfite Reductase

P-582-type sulfite reductase has only been reported in several species of the spore-
forming SRB genera Desulfosporosinus and Desulfotomaculum [ 15 , 25 , 65 ,
111 ]. The structure of dSiR from Dst. thermocisternum, a Gram-positive thermo-
philic sulfate reducer, has been deduced from gene analysis [ 112 ]. From cloning
and sequence analysis, it is suggested that the dsr operon in Dst. thermocisternum is
similar to that of Ar. fulgidus and D. vulgaris H in terms of sequence and gene
organization.
The dsrA gene encodes for a protein of 54.1 kDa with a 76 % similarity to Ar.
fulgidus DsrA and an 83 % similarity to DsrA of D. vulgaris H. Down-stream of the
dsrA gene in Dst. thermocisternum is the dsrB gene and it encodes a protein of