Abstract
Anaerobic photosynthetic green sulfur bacteria utilize sulfur compounds such as sulfide and thiosulfate as electron donors of photosynthesis. Previously, we reported purification of four proteins (SoxYZ, SoxB, SoxAX-CT1020, SoxJ) involved in thiosulfate oxidation from Chlorobaculum tepidum cells. Some thiosulfate oxidizing bacteria have CT1020 homolog, named soxK, but its function was unknown. We have over-expressed SoxA, SoxX and SoxK in E. coli, and found that SoxK strengthens the binding of SoxA and SoxK, and stimulates the activity. Accordingly, SoxK is designated as SAXB (SoxAB-binding protein). From comparative genome analyses, about one third of thiosulfate bacteria have soxK homologues, and the deduced SoxA and SoxX proteins cluster together according to the presence and absence of soxK. Flavoprotein SoxJ is a homologue of flavoprotein subunit of flavocytochrome c. SoxJ itself has no thiosulfate oxidizing activity, but it accelerated the reaction rate about twice when added to the mixture of SoxAXK, SoxB and SoxYZ.
