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 novell us) has not been established beyond doubt. Recently, isolation of the soi A B genes, which encode a soluble sulfite:cytochrome c oxidoreductase, has be en reported, indicating that a thiosulfate-oxidizing pathway not involving a multienzyme complex may exist in this organism. Here we report the clonin g and sequencing of the soxBCD genes from S. novella, which are closely rel ated to the corresponding genes encoding the thiosulfate-oxidizing multienz yme complex from Paracoccus pantotrophus. These findings suggest two distin ct pathways for thiosulfate oxidation in S. novella. The expression of sol AB and soxC in cells grown on thiosulfate- and/or glucose-containing media was studied by Western blot analysis. The results showed that the SorAB pro tein is synthesized in the presence of thiosulfate irrespective of the pres ence of glucose. In contrast, the SoxC protein is subject to repression by glucose; the repression, however, appears to be dependent on the relative a mounts of glucose and thiosulfate present. The regulatory effects observed for the expression of sorAB are likely to be mediated by an extracytoplasmi c function sigma factor encoded by the sigE gene identified upstream of sor AB.