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44.2 kDa with an 89 % similarity to the Ar. fulgidus DsrB . Down-stream from the
dsrB is the dsrD gene which encodes for a peptide of 11.1 kDa and the DsrD, a
hypothetical
-polypeptide, has a projected sequence identity of 38 % with Ar.
fulgidus DsrD and 41 % identity to D. vulgaris H DsrD. The proposed operon for
dissimilatory sulfite reductase in Dst. thermocisternum consists of dsrABD.
ʴ
2.4.1.3 Desulforubidin-Type Sulfite Reductase
The red brown protein, desulforubidin, belongs to the genera Desulfohalobium ,
Desulfosarcina , Desulfomicrobium , Desulfocurvus, Desulfobulbus, Desulfofustis ,
and Desulfobacter [ 2 , 39 , 113 , 114 ]. The crystal structure at 2.5
resolution of the
Desulfomicrobium ( Dsm. ) norvegicum dSiR was reported by Oliveira et al. [ 93 ].
Due to the distinctive spectral characteristics, the dSiR is a desulforubidin-type with
the
Å
ʱ 2 ʲ 2 ʳ 2 structure and the isolated complex contains two DsrC proteins. The Dsm.
norvegicum dSiR contains four siroheme groups and eight [4Fe-4S] clusters.
One of the cofactors in the dSiR is a siroheme bound to a [4Fe-4S] cluster and
each of the peptides making up the
ʱ 2 ʲ 2 structure contains a siroheme-[4Fe-4S]
moiety. The siroheme-[4Fe-4S] component is at the interface of the DsrA and DsrB
structures with the [4Fe-4S] cluster bound into the DsrB subunit. It is suggested by
Oliveira et al. [ 93 ] that the two siroheme-[4Fe-4S] cofactors bound in the DsrA
structures are not involved in the enzymatic sulfite reduction because they are not
readily accessible. In comparison, the site where the possibly inactive siroheme-
[4Fe-4S] is bound in Dsm. norvegicum is occupied by a sirohydrochlorin in
D. vulgaris H and D. gigas . The DsrC of Dsm. norvegicum has a helical structure
where the C-terminal segment of the protein is flexible and is inserted in the cleft
between DsrA and DsrB.
The dSiR produced by Desulfobacter ( Dba. ) vibrioformis and Desulfobulbus
( Dbu. ) rhabdoformis is of the desulforubidin-type and the operons for dSiR were
characterized by gene analysis [ 115 ]. As in the case of dSiR from the genera
Desulfovibrio, Desulfotomaculum , Desulfomicrobium, and Archaeoglobus, the
ʱ
and
subunits of dSiR in Dba. vibrioformis and Dbu. rhabdoformis are encoded on
dsrA and dsrB , respectively. The polypeptide encoded on dsrA is 48-50 kDa and for
dsrB is 42-43 kDa in Dba. vibrioformis and Dbu. rhabdoformis , respectively, and is
comparable to polypeptides reported for other siroheme-containing sulfite reduc-
tases. Important characteristics of siroheme-containing dSiR are the homology
region of H1-H5 and the binding site for the ferredoxin motif [ 116 , 117 ] and
these are also present in Dba. vibrioformis and Dbu. rhabdoformis. High sequence
identity was reported when dsrA and dsrB from Dba. vibrioformis and Dbu.
rhabdoformis were compared to respective genes found in D. vulgaris H , Dst.
thermocisternum. Ar.
ʲ
fulgidus, Ar.
profundus, Pyb.
islandicum,
and
Allochromatium ( Alc. ) vinosum.
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