SoxAX Binding Protein, a Novel Component of the Thiosulfate-Oxidizing Multienzyme System in the Green Sulfur Bacterium Chlorobium tepidum

Abstract

From the photosynthetic green sulfur bacterium Chlorobium tepidum (pro synon. Chlorobaculum tepidum), we have purified three factors indispensable for the thiosulfate-dependent reduction of the small, monoheme cytochrome c 554 . These are homologues of sulfur-oxidizing (Sox) system factors found in various thiosulfate-oxidizing bacteria. The first factor is SoxYZ that serves as the acceptor for the reaction intermediates. The second factor is monomeric SoxB that is proposed to catalyze the hydrolytic cleavage of sulfate from the SoxYZ-bound oxidized product of thiosulfate. The third factor is the trimeric cytochrome c 551 , composed of the monoheme cytochrome SoxA, the monoheme cytochrome SoxX, and the product of the hypothetical open reading frame CT1020. The last three components were expressed separately in Escherichia coli cells and purified to homogeneity. In the presence of the other two Sox factors, the recombinant SoxA and SoxX showed a low but discernible thiosulfate-dependent cytochrome c 554 reduction activity. The further addition of the recombinant CT1020 protein greatly increased the activity, and the total activity was as high as that of the native SoxAX-CT1020 protein complex. The recombinant CT1020 protein participated in the formation of a tight complex with SoxA and SoxX and will be referred to as SAXB (SoxAX binding protein). Homologues of the SAXB gene are found in many strains, comprising roughly about one-third of the thiosulfate-oxidizing bacteria whose sox gene cluster sequences have been deposited so far and ranging over the Chlorobiaciae, Chromatiaceae, Hydrogenophilaceae, Oceanospirillaceae, etc. Each of the deduced SoxA and SoxX proteins of these bacteria constitute groups that are distinct from those found in bacteria that apparently lack SAXB gene homologues.

Sulfur globule oxidation in green sulfur bacteria is dependent on the dissimilatory sulfite reductase system

“…Thiosulfate oxidation in C. tepidum is performed by the SOX system (Azai et al, 2009;Ogawa et al, 2008), but is inhibited in the presence of sulfide (Chan et al, 2008b). In our C. tepidum wild-type cultures, thiosulfate accumulated under sulfide-oxidizing conditions and was subsequently consumed when sulfide was depleted (Fig.…”

“…In C. tepidum, the membrane-bound SQR homologues CT0117 and CT1087 are responsible for some of the sulfide oxidation activity in the cells (Chan et al, 2009). Thiosulfate oxidation in GSB is carried out by the sulfuroxidizing (SOX) system in a manner similar to that in A. vinosum (Azai et al, 2009;Ogawa et al, 2008). In the proposed pathway the sulfone moiety of thiosulfate is oxidized to sulfate and released, whereas the sulfane moiety is released to the putative pool of sulfur globule precursors and subsequently either deposited in sulfur globules or oxidized to sulfite by the dsr system (Fig.…”

Biochemical Studies of asoxF-Encoded Monomeric Flavoprotein Purified from the Green Sulfur BacteriumChlorobaculum tepidumThat Stimulatesin VitroThiosulfate Oxidation

“…The soxK gene occurs within the sox cluster of some bacteria, such as Chlorobiaceae 11,12) and Chromatiaceae, but does not appear to occur in the genomes of bacteria such as P. pantotrophus and S. novella. 20) SoxK from C. tepidum was found to participate in binding and enhancing of the function of SoxA and SoxX, and hence was designated SAXB (SoxAX binding protein). 20) The soxF and soxE genes encode the homologs of flavoprotein and the cytochrome moiety respectively of the dimeric flavocytochrome c (FCSD or FCC) complex, 2,18) which has been reported to act as a sulfide dehydrogenase in Chlorobium limicola f. sp.…”

“…20) SoxK from C. tepidum was found to participate in binding and enhancing of the function of SoxA and SoxX, and hence was designated SAXB (SoxAX binding protein). 20) The soxF and soxE genes encode the homologs of flavoprotein and the cytochrome moiety respectively of the dimeric flavocytochrome c (FCSD or FCC) complex, 2,18) which has been reported to act as a sulfide dehydrogenase in Chlorobium limicola f. sp. thiosulfatophilum.…”

Modelling sponge-symbiont metabolism

“…This is the same as free-living, symbiotic (Kleiner et al, 2012a;Muyzer et al, 2011a,b) and phototrophic SOB (Friedrich et al, 2005;Ogawa et al, 2008). The complete Sox system yields 8 electrons per thiosulfate molecule, but the loss of the soxCD enzyme results in just 2 electrons being generated and the formation of sulfur globules (Fazzini et al, 2013).…”