The purple sulfur photosynthetic bacterium Thiocapsa roseopersicina BBS contains a heat-stable membrane-associated hydrogenase encoded by the hyn operon. Expression from the hyn operon regulatory region is up-regulated under anaerobic conditions. cis elements were mapped between positions -602 and -514 upstream from the hynS gene. Within this region two sequences that resemble DNA sites for FNR were recognized. The gene of an FNR homologue, FnrT, was identified in the genome of T. roseopersicina, and an fnrT knockout mutant was constructed. Anaerobic induction of hynS expression was abolished in the fnrT mutant, suggesting that FnrT is an activator of the hynS promoter. The T. roseopersicina hynS promoter could be activated in Escherichia coli, and this regulation was dependent on E. coli FNR. In vitro experiments with purified E. coli Ala154 FNR protein and purified E. coli RNA polymerase showed that FNR bound to two sites in the hyn regulatory region, that FNR could activate transcription initiation at the hynS promoter, and that FNR bound at the two target sites activated to different extents.