Molecular biology studies of the uptake hydrogenase of Rhodobacter capsulatus and Rhodocyclus gelatinosus

In the photosynthetic bacteria, as in other N2-fixing bacteria, two main enzymes are involved in H2 metabolism: nitrogenase, which catalyses the photoproduction of H2, and a membrane-bound (NiFe) hydrogenase, which functions as an H2-uptake enzyme. The structural genes for Rhodobacter capsulatus and Rhodocyclus gelatinosus uptake hydrogenases were isolated and sequenced. They present the same organization, with the gene encoding the small subunit (hupS) (molecular masses 34.2 and 34.6 kDa, respectively) preceding the gene encoding the large one (hupL) (molecular masses 65.8 and 68.5 kDa, respectively). The two hupSL genes apparently belong to the same operon. The deduced protein sequences of the small and of the large subunits share nearly 80% and maximally 70% identity, respectively, with their counterparts in uptake hydrogenases found in N2-fixing bacteria. However, unlike in Bradyrhizobium japonicum, R. gelatinosus or Azotobacter chroococcum, another open reading frame (ORFX) was found downstream and contiguous to the R. capsulatus hupSL whose transcription seemed to depend on the same hup promoter as hupSL. ORFX contained 786 nucleotides capable of encoding a hydrophobic polypeptide of 262 amino acids (30.2 kDa).