HupO, a Novel Regulator Involved in Thiosulfate-Responsive Control of HupSL [NiFe]-Hydrogenase Synthesis in Thiocapsa roseopersicina

ABSTRACT[NiFe]-hydrogenases are regulated by various factors to fulfill their physiological functions in bacterial cells. The photosynthetic purple sulfur bacteriumThiocapsa roseopersicinaharbors four functional [NiFe]-hydrogenases: HynSL, HupSL, Hox1, and Hox2. Most of these hydrogenases are functionally linked to sulfur metabolism, and thiosulfate has a central role in this organism. The membrane-associated Hup hydrogenases have been shown to play a role in energy conservation through hydrogen recycling. The expression of Hup-type hydrogenases is regulated by H2inRhodobacter capsulatusandCupriavidus necator; however, it has been shown that the corresponding hydrogen-sensing system is nonfunctional inT. roseopersicinaand that thiosulfate is a regulating factor ofhupexpression. Here, we describe the discovery and analysis of mutants of a putative regulator (HupO) of the Hup hydrogenase inT. roseopersicina. HupO appears to mediate the transcriptional repression of Hup enzyme synthesis under low-thiosulfate conditions. We also demonstrate that the presence of the Hox1 hydrogenase strongly influences Hup enzyme synthesis in thathupexpression was decreased significantly in thehox1mutant. This reduction in Hup synthesis could be reversed by mutation ofhupO, which resulted in strongly elevatedhupexpression, as well as Hup protein levels, and concomitantin vivohydrogen uptake activity in thehox1mutant. However, this regulatory control was observed only at low thiosulfate concentrations. Additionally, weak hydrogen-dependenthupexpression was shown in thehupOmutant strain lacking the Hox1 hydrogenase. HupO-mediated Hup regulation therefore appears to link thiosulfate metabolism and the hydrogenase network inT. roseopersicina.