Evidence for two pathways of thiosulfate oxidation in Starkeya novella (formerly Thiobacillus novellus)

The pathway of thiosulfate oxidation in the facultatively chemolithotrophic, sulfur-oxidizing bacterium Starkeya novella (formerly Thiobacillus novellus) has not been established beyond doubt. Recently, isolation of the sorAB genes, which encode a soluble sulfite:cytochrome c oxidoreductase, has been reported, indicating that a thiosulfate-oxidizing pathway not involving a multienzyme complex may exist in this organism. Here we report the cloning and sequencing of the soxBCD genes from S. novella, which are closely related to the corresponding genes encoding the thiosulfate-oxidizing multienzyme complex from Paracoccus pantotrophus. These findings suggest two distinct pathways for thiosulfate oxidation in S. novella. The expression of sorAB and soxC in cells grown on thiosulfate- and/or glucose-containing media was studied by Western blot analysis. The results showed that the SorAB protein is synthesized in the presence of thiosulfate irrespective of the presence of glucose. In contrast, the SoxC protein is subject to repression by glucose; the repression, however, appears to be dependent on the relative amounts of glucose and thiosulfate present. The regulatory effects observed for the expression of sorAB are likely to be mediated by an extracytoplasmic function sigma factor encoded by the sigE gene identified upstream of sorAB.