UNUSUAL GENE ARRANGEMENT OF THE BIDIRECTIONAL HYDROGENASE AND FUNCTIONAL-ANALYSIS OF ITS DIAPHORASE SUBUNIT HOXU IN RESPIRATION OF THE UNICELLULAR CYANOBACTERIUM ANACYSTIS-NIDULANS

The bidirectional, NAD(+)-dependent hydrogenase from cyanobacteria is encoded by the structural genes hoxFUYH, which have been found to be c lustered, though interspersed with different open reading frames (ORFs ), in the heterocystous, N-2-fixing Anabaena variabilis and in the uni cellular Synechocystis PCC 6803. In another unicellular, non N-2-fixin g cyanobacterium, Anacystis nidulans, hoxF has now been identified as being separated by at least 16 kb from the residual structural genes h oxUYH. An ORF (termed hoxE gene) is located immediately upstream of ho xF in A. nidulans and in Synechocystis. Its deduced amino acid sequenc e shows similarities to the NuoE subunit of NADH dehydrogenase I of E. coli, to the homologous subunit of respiratory complex I in mitochond ria, and also to the first 104 amino acids of HoxF in A. nidulans and Synechocystis. The diversity in the arrangement of hydrogenase genes i n cyanobacteria is puzzling. The subunits HoxE, HoxF and HoxU of the d iaphorase part of the bidirectional hydrogenase have been discussed to be shared both by respiratory complex I and bidirectional hydrogenase in cyanobacteria. Different hoxU mutants were obtained by inserting a lacZKm(R) cassette into the gene both in A. nidulans and Anacystis PC C 7942. Such mutants showed reduced Hz-evolution activities catalyzed by the bidirectional hydrogenase, but had nonimpaired respiratory O-2- uptake. A common link between respiratory complex I and the diaphorase part of the bidirectional hydrogenase in cyanobacteria may still exis t, but this hypothesis could not be verified in the present study by a nalyzing defined mutants impaired in one of the diaphorase genes.