DISSIMILATORY SULFITE REDUCTASE FROM ARCHAEOGLOBUS-FULGIDUS - PHYSICOCHEMICAL PROPERTIES OF THE ENZYME AND CLONING, SEQUENCING AND ANALYSISOF THE REDUCTASE GENES
C. Dahl et al., DISSIMILATORY SULFITE REDUCTASE FROM ARCHAEOGLOBUS-FULGIDUS - PHYSICOCHEMICAL PROPERTIES OF THE ENZYME AND CLONING, SEQUENCING AND ANALYSISOF THE REDUCTASE GENES, Journal of General Microbiology, 139, 1993, pp. 1817-1828
A dissimilatory sulphite reductase was isolated from the extremely the
rmophilic dissimilatory sulphate-reducing archaeon Archaeoglobus fulgi
dus. In common with other dissimilatory sulphite reductases thus far c
haracterized, the enzyme has an alpha2beta2-structure and contains sir
ohaem, non-haem iron atoms and acid labile sulphide. The oxidized enzy
me exhibited absorption maxima at 281, 394, 545 and 593 nm with a weak
band around 715 nm. We have cloned and sequenced the genes for the al
pha and beta subunits of this enzyme, which we designate dsrA and dsrB
, respectively. They are contiguous in the order dsrA dsrB and probabl
y comprise an operon, since dsrA is preceded by sequences characterist
ic of promoters in methanogenic archaea, and dsrB is followed by a seq
uence resembling termination signals in extremely thermophilic sulphur
-dependent archaea. dsrA and dsrB encode 47-4 kDa and 41.7 kDa peptide
s, which have 25.6 % amino acid sequence identity, indicating that the
y may have arisen by duplication of an ancestral gene. Each deduced pe
ptide contains cysteine clusters resembling those postulated to bind s
irohaem-[Fe4S4] complexes in sulphite reductases and nitrite reductase
s from other species. The dsrB encoded peptide lacks a single cysteine
residue in one of the two clusters, suggesting that only the alpha su
bunit binds a sirohaem-[Fe4S4] complex, and chemical analyses showed t
he presence of only two sirohaems per alpha2beta2 enzyme molecule. Bot
h deduced peptides also contain an arrangement of cysteine residues ch
aracteristic of [Fe4S4] ferredoxins, and chemical analyses were consis
tent with the presence of six [Fe4S4] clusters per alpha2beta2 enzyme
molecule, two of which would be expected to be associated with sirohae
m while the other four could bind to the ferredoxin-like sites.