Abstract
Anabaena variabilis ATCC 29413 is a filamentous heterocystous cyanobacterium that fixes nitrogen under a variety of environmental conditions. Under aerobic growth conditions, nitrogen fixation depends upon differentiation of heterocysts and expression of either a Mo-dependent nitrogenase or a V-dependent nitrogenase in those specialized cells. Under anaerobic conditions, a second Mo-dependent nitrogenase gene cluster, nifII, was expressed in vegetative cells long before heterocysts formed. A strain carrying a mutant gene in the nifII cluster did not fix nitrogen under anaerobic conditions until after heterocysts differentiated. The nifII cluster was similar in organization to the nifI cluster that is expressed in heterocysts and that includes nifBSUHDKENXW as well as three open reading frames that are conserved in both cyanobacterial nif clusters.
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