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

Abstract

b[NiFe]-hydrogenases are regulated by various factors to fulfill their physiological functions in bacterial cells. The photosynthetic purple sulfur bacterium Thiocapsa roseopersicina harbors 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 H 2 in Rhodobacter capsulatus and Cupriavidus necator; however, it has been shown that the corresponding hydrogen-sensing system is nonfunctional in T. roseopersicina and that thiosulfate is a regulating factor of hup expression. Here, we describe the discovery and analysis of mutants of a putative regulator (HupO) of the Hup hydrogenase in T. 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 that hup expression was decreased significantly in the hox1 mutant. This reduction in Hup synthesis could be reversed by mutation of hupO, which resulted in strongly elevated hup expression, as well as Hup protein levels, and concomitant in vivo hydrogen uptake activity in the hox1 mutant. However, this regulatory control was observed only at low thiosulfate concentrations. Additionally, weak hydrogen-dependent hup expression was shown in the hupO mutant strain lacking the Hox1 hydrogenase. HupO-mediated Hup regulation therefore appears to link thiosulfate metabolism and the hydrogenase network in T. roseopersicina. Hydrogenases are ancient metalloenzymes that catalyze the reversible oxidation of molecular hydrogen. They can be found in many bacteria and archaea, as well as in eukaryotic microalgae. Three major groups of hydrogenases are distinguished according to their metal content: the [FeFe]-hydrogenases, [NiFe]-hydrogenases, and the iron-sulfur-cluster-free hydrogenase enzymes (1-3). The [NiFe]-hydrogenases contain a large (60-to 65-kDa) catalytic subunit and a small electron transfer subunit (25 to 35 kDa). Additional proteins are required for the posttranslational maturation of the enzyme. These accessory proteins participate in a complex multistep assembly process of the core catalytic center with well-defined specific functions (4-6). The [NiFe]-hydrogenases can be further classified according to their localization, function, and possible associated subunits. Four major groups have been distinguished: the membrane-associated uptake hydrogenases, the hydrogen-sensing hydrogenases, the bidirectional NADP/NAD-reducing mostly cytoplasmic hydrogenases, and the energy-converting membrane-associated hydrogen-evolving hydrogenases (3). A number of bacteria possess more than one [NiFe]-hydrogenase enzyme (e.g., Escherichia coli, Cupriavidus necator [formerly Ralstonia eutropha], and Thiocap...

Hup-Type Hydrogenases of Purple Bacteria: Homology Modeling and Computational Assessment of Biotechnological Potential

“…For example, it was shown to lose its activity irreversibly under air in just a day [27]. Since then, it has become mostly the subject of basic research exploring regulation of its synthesis [53,54], but, from there, some interesting information could arise. Several works demonstrated the susceptibility of Hup-hydrogenases toward cold denaturation ( [43,53,55] p. 83).…”