In Hyphomicrobium denitrificans two related sulfane-sulfur responsive transcriptional repressors regulate thiosulfate oxidation and have a deep impact on nitrate respiration and anaerobic biosyntheses

Bacteria have evolved multiple strategies to sense and respond to the availability of inorganic reduced sulfur compounds such as thiosulfate. In Hyphomicrobium denitrificans , an obligately chemoorganoheterotrophic Alphaproteobacterium, the use of thiosulfate as a supplemental electron donor is regulated by two homologous sulfane-sulfur-responsive ArsR-type transcriptional repressors, sHdrR and SoxR. Here, we provide information on the distribution and phylogeny of sHdrR, the relevance of its two conserved cysteines i n vivo , and identify the genes controlled by SoxR and sHdrR not only by targeted qRT-PCR but also by global RNA-Seq-based analyses of regulator-deficient mutant strains.The absence of sHdrR and SoxR affected 165 and 170 genes, respectively, with 138 genes overlapping. SoxR affects the sox genes for periplasmic thiosulfate oxidation and sulfane sulfur import into the cytoplasm, as well as the lip-shdr-lbpA genes encoding the cytoplasmic enzymes essential for sulfite formation. sHdrR affects only a subset of these genes. The transcription of sox genes remains unaltered in its absence. sHdrR and SoxR act cooperatively, possibly involving heterodimer formation, and their activity also involves interaction with other transcriptional regulators. Most importantly, sHdrR/SoxR regulation extends far beyond sulfur oxidation and deeply affects anaerobic metabolism, particularly denitrification in H. denitrificans .