The expression of many membrane bound [NiFe] hydrogenases is regulated by their substrate molecule, hydrogen. The HupSL hydrogenase, encoded in the hupSLCDHIRoperon, probably plays a role in hydrogen recycling in the phototrophic purple bacterium, Thiocapsa roseopersicina BBS. RpoN, coding for sigma factor 54, was shown to be important for expres-sion, suggesting a regulated biosynthsis from the hup gene cluster.