The third subunit of desulfoviridin‐type dissimilatory sulfite reductases

Abstract

(Kcccived October 25, 1991) -EJB 91 1438In addition to the 50-kDa (a) and 40-kDa (0) subunits, a n 11-kDa polypeptide has been discovered in highly purified Desulfovihrio vulguris (Hildenborough) dissimilatory sulfite reductase. This is in contrast with the hitherto generally accepted u2p2 tetrameric subunit composition. Purification, high-ionic-strength gel-filtration, native electrophoresis and isoelectric focussing d o not result in dissociation of the 11-kDa polypeptide from the complex. Densitometric scanning of SDS gels and denaturing gel-filtration indicate a stoichiometric occurrence. A similar 1 I-kDa polypeptide is present in the desulfoviridin of D. vulgaris oxarnicus (Monticello), D . gigas and D. dcsulfuricuns ATCC 27774. We attribute an cc,D,y, subunit structure to desulfoviridin-type sulfite reductases. N-terminal sequences of the a, / 3 and y subunits are reported.A key enzyme in dissimilatory sulfate reduction is sulfite reductase, a complex redox enzyme containing both Fe/S and siroheme prosthetic groups. Sulfite reductases that are supposed to have a dissimilatory function have been observed in, and isolated from, over 20 species of microorganisms [ l -41. Kinetic parameters and subunit structure of dissimilatory sulfite reductases are markedly different from assimilatory sulfite reductases.Dissimilatory enzymes are x 2 P 2 proteins (1 50-230 kDa) that have a millimolar K,,, for sulfite, a slightly acidic pH optimum and reduce their substrate to SJOzSp and S20:-(1, 4-81, The physiological relevance of the in vitro observed product composition is a matter of controversy. A number of explanations has been put forward: loss of interaction with the cytoplasmic membrane [9], in vitro assay conditions [lo], and the possibility that the observed products are in vivo intermediates [I 11. Partial demctallation of the siroheme is observed in desulfoviridins [2,12] and is presumably an intrinsic feature of some dissimilatory enzymes. It probably has no bearing on the formation of S 3 0 i -and SzO:-since desulfoviridin, desulfofuscidin, P582 and desulforubidin-type dissimilatory enzymes have a comparable product composition and specific activity [2-7, 131.Contrarily, assimilatory sulfite reductases cleanly perform the full six-electron reduction to sulfide. They have a slightly basic pH optimum with a submillimolar K,, for sulfite. The subunit composition, however, is non-uniform: ugp4 [14], a4-6 1351, or cc2 [16]. This probably reflects differences in the source of reducing equivalents.A distinct, third, group of sulfitc reductases comprises the so-called low-molecular-mass sulfite reductases. These enzymes arc presumably assimilatory. They differ from the other two groups in two aspects: they are monomeric (20 -30 kDa), We report here that D . vulgaris (Hildenborough) desulfoviridin contains a small, hitherto unnoted, y-subunit in addition to the usual a2P2 motive of dissimilatory enzymes. Nterminal sequences, size and stoichiometry of the subunits have been determined. An immunological comparison of d...

Expression of the γ‐subunit gene of desulfoviridin‐type dissimilatory sulfite reductase and of the α‐ and β‐subunit genes is not coordinately regulated

“…This project began with the premise that the subunit structure was alp2, and with the expectation of finding an operon of about 2.5 kb, encoding both the a (50 kDa) and p(40 kDa) subunits. However, the cloning of a single gene for an 11-kDa protein reported in this paper presented a puzzle that was solved by the discovery of the y subunit by Pierik et al [2]. …”

“…However, recent evidence demonstrated the presence of a third y subunit with a proposed c(z,! ?2y2 subunit structure [2]. At present the complete amino acid sequences of these subunits are not known, although their NHz-terminal sequences have been published [2].…”

“…Dissimilatory sulfite reductases are by definition found in the sulfate-reducing bacteria, where they function as the terminal oxidase in the sulfate-reduction pathway [I]. Four different dissimilatory sulfite reductases have so far been distinguished : desulfoviridin is the predominant enzyme in Desulfovibrio species [2], desulforubidin has been isolated from Desulfovibrio desulfuricans Norway [3] and desulfofuscidin from Thermodesulfobacterium commune [4], whereas P582 is present in the Gram-positive sulfate reducer Desulfotomaculum nigrificans [5]. The number of different sulfite reductases could be larger in view of the divergence of genera of sulfate-reducing bacteria established by nutritional studies [6] and 16s rRNA sequencing [7, 81.…”

Post-Translational Modifications of Desulfovibrio vulgaris Hildenborough Sulfate Reduction Pathway Proteins

“…It is therefore interesting to speculate that the purpose of 14 these modified lysines may be to stabilize the charged face of the helix. The DsrC 15 enzyme in D. vulgaris forms part of the DsrABC complex 54 , although this may not be the 16 case in the A. fulgidus 50 . DsrC is also predicted to interact with the DsrMKJOP 17 complex [54][55][56] .…”

“…The DsrC 15 enzyme in D. vulgaris forms part of the DsrABC complex 54 , although this may not be the 16 case in the A. fulgidus 50 . DsrC is also predicted to interact with the DsrMKJOP 17 complex [54][55][56] . As both the PTMs point outward rather than inward, a modification at 18 either of these positions may impact protein-protein interactions 57 …”