CLONING, MOLECULAR ANALYSIS AND INSERTIONAL MUTAGENESIS OF THE BIDIRECTIONAL HYDROGENASE GENES FROM THE CYANOBACTERIUM ANACYSTIS-NIDULANS

Among cyanobacteria, the heterocystous, N-2-fixing Anabaena variabilis and the unicellular Anacystis nidulans have recently been shown to po ssess an NAD(+)-dependent, bidirectional hydrogenase, A 5.0 kb DNA seg ment of the A. nidulans genome is now identified to harbor the structu ral genes hoxUYH coding for three subunits of the bidirectional hydrog enase. The gene arrangement in A. nidulans and in A. variabilis is rem arkably dissimilar, In A. nidulans, but not in A. variabilis, the four accessory genes hoxW, hypA, hypB and hypF could be identified downstr eam of hoxH. An insertional homozygous mutant in hoxH from A. nidulans was completely inactive in performing Na2S2O4-dependent H-2 evolution but could utilize the gas with almost 50% of the activity of the wild type, These findings with the first defined hydrogenase mutant in any photosynthetic, O-2-evolving microorganism indicate that the unicellu lar cyanobacterium A. nidulans possesses both an uptake and a bidirect ional hydrogenase, The physiological role(s) of the two hydrogenases i n unicellular non-N-2-fixing cyanobacteria is not yet understood.