Hydrogenases in the Cyanobacterium Nostoc sp. Strain PCC 73102

Abstract: Many cyanobacteria are capable of fixing atmospheric N2, a reaction that is also producing H2. This hydrogen is often metabolized by an enzyme called hydrogenase. The aim of the present work was to identify and characterize hydrogen metabolism/hydrogenases in the free-living N2-fixing cyanobacterium Nostoc sp.strain PCC 73102. With immunological techniques it was revealed that Nostoc PCC 73102 contains proteins that are immunologically related to hydrogenases from other microorganisms and that these proteins seem to be present in both the vegetative cells and the heterocysts. A light-dependent hydrogen uptake was detected in nitrogen-fixing cells of Nostoc PCC 73102. This hydrogen uptake is positively regulated by the substrate H2. Furthermore, the in vivo nitrogenase and uptake hydrogenase activities appear to be co-regulated when nitrogen-fixing cells are exposed to either combined nitrogen or organic carbon sources. Using both physiological and molecular techniques, no evidence could be found for the presence of a reversible/bidirectional hydrogenase in Nostoc PCC 73102. The characterization of the structural genes encoding the uptake hydrogenase in Nostoc PCC 73102 demonstrated a high sequence identity with the sequence of the cyanobacterium Anabaena PCC 7120, but a high degree of difference in comparison with sequences from other microorganisms. In addition, Nostoc PCC 73102 appears not to perform a site specific rearrangement within hupL. Furthermore, in a Nostoc strain containing both an uptake- and a bidirectional hydrogenase, an induction of a hupL transcript was evident after a shift from non-nitrogen fixing to nitrogen fixing conditions.