CHARACTERIZATION OF GENES FOR A 2ND MO-DEPENDENT NITROGENASE IN THE CYANOBACTERIUM ANABAENA-VARIABILIS

Anabaena variabilis ATCC 29413 is a filamentous heterocystous cyanobac terium that fixes nitrogen under a variety of environmental conditions . Under aerobic growth conditions, nitrogen fixation depends upon diff erentiation of heterocysts and expression of either a Mo-dependent nit rogenase or a V-dependent nitrogenase in those specialized cells. Unde r anaerobic conditions, a second Mo-dependent nitrogenase gene cluster , nifII, was vegetative cells long before heterocysts formed. A strain carrying a mutant gene In the nifII cluster did not fis nitrogen unde r anaerobic conditions until after heterocysts differentiated. The nif II cluster was similar in organization to the nifI cluster that is exp ressed in heterocysts and that includes nifBSUHDKENXW as well as three open reading frames that are conserved in both cyanobacterial nif clu sters.