UNUSUAL ORGANIZATION OF THE GENES-CODING FOR HYDSL, THE STABLE [NIFE]HYDROGENASE IN THE PHOTOSYNTHETIC BACTERIUM THIOCAPSA-ROSEOPERSICINA BBS

The characterization of a hyd gene cluster encoding the stable, bidire ctional [NiFe]hydrogenase 1 enzyme in Thiocapsa roseopersicina BBS, a purple sulfur photosynthetic bacterium belonging to the family Chromat iaccae, is presented, The heterodimeric hydrogenase 1 had been purifie d to homogeneity and thoroughly characterized (K. L. Kovacs et al., J, Biol, Chem, 266:947-951, 1991; C, Bagyinka et al., J, Am. Chem, Sec. 115:3567-3585, 1993), As an unusual feature, a 1,979-bp intergenic seq uence (IS) separates the structural genes hydS and hydL, which encode the small and the large subunits, respectively. This IS harbors two se quential open reading frames (ORFs) which may code for electron transf er proteins ISP1 and ISP2. ISP1 and ISP2 are homologous to ORF5 and OR F6 in the hmc operon, coding for a transmembrane electron transfer com plex in Desulfovibrio vulgaris. Other accessory proteins are not found immediately downstream or upstream of hydSL. A hup gene cluster codin g for a typical hydrogen uptake [NiFe] hydrogenase in T, roseopersicin a was reported earlier (A, Colbeau et al. Gene 140:25-31, 1994). The d educed amino acid sequences of the two small (hupS and hydS) and large subunit (hupL and hydL) sequences share 46 and 58% identity, respecti vely, The hup and hyd genes differ in the arrangement of accessory gen es, and the genes encoding the two enzymes are located at least 15 kb apart on the chromosome, Both hydrogenases are associated with the pho tosynthetic membrane, A stable and an un stable hydrogenase activity c an be detected in cells grown under nitrogen-fixing conditions; the la tter activity is missing in cells supplied with ammonia as the nitroge n source, The apparently constitutive and stable activity corresponds to hydrogenase 1, coded by hydSL, and the inducible and unstable secon d hydrogenase mag be the product of the hup gene cluster.