CLONING AND NUCLEOTIDE-SEQUENCES OF THE GENES FOR THE SUBUNITS OF NAD-REDUCING HYDROGENASE OF ALCALIGENES-EUTROPHUS H16

The genes hoxF, -U, -Y, and -H which encode the four subunit polypeptides α, γ, δ, and β of the NAD-reducing hydrogenase (HoxS) of Alcaligenes eutrophus H16, were cloned, expressed in Pseudomonas facilis, and sequenced. On the basis of the nucleotide sequence, the predicted amino acid sequences, and the N-terminal amino acid sequences, it was concluded that the structural genes are tightly linked and presumably organized as an operon, denoted hoxS. Two pairs of -24 and -12 consensus sequences resembling RpoN-activatable promoters lie upstream of hoxF, the first of the four genes. Primer extension experiments indicate that the second promoter is responsible for hoxS transcription. hoxF and hoxU code for the flavin-containing dimer (α and γ subunits) of HoxS which exhibits NADH:oxidoreductase activity. A putative flavin-binding region is discussed. The 26.0-kilodalton (kDa) γ subunit contains two cysteine clusters which may participate in the coordination of two [4Fe-4S] centers. The genes hoxY and hoxH code for the small 22.9-kDa δ subunit and the nickel-containing 54.8-kDa β subunit, respectively, of the hydrogenase dimer of HoxS. The latter dimer exhibits several conserved regions found in all nickel-containing hydrogenases. The roles of these regions in coordinating iron and nickel are discussed. Although the deduced amino acid sequences of the δ and β subunits share some conserved regions with the corresponding polypeptides of other [NiFe] hydrogenases, the overall amino acid homology is marginal. Nevertheless, significant sequence homology (35%) to the corresponding polypeptides of the soluble methylviologen-reducing hydrogenase of Methanobacterium thermoautotrophicum was found. Unlike the small subunits of the membrane-bound and soluble periplasmic hydrogenases, the HoxS protein does not appear to be synthesized with an N-terminal leader peptide.