A 2ND NITROGENASE IN VEGETATIVE CELLS OF A HETEROCYST-FORMING CYANOBACTERIUM

In many filamentous cyanobacteria nitrogen fixation occurs in differen tiated cells called heterocysts. Filamentous strains that do not form heterocysts may fix nitrogen in vegetative cells, primarily under anae robic conditions. We describe here two functional Mo-dependent nitroge nases in a single organism, the cyanobacterium Anabaena variabilis, Us ing a lacZ reporter with a fluorescent P-galactoside substrate for in situ localization of gene expression, we have shown that the two clust ers of nif genes are expressed independently, One nitrogenase function s only in heterocysts under either aerobic or anaerobic growth conditi ons, whereas the second nitrogenase functions only under anaerobic con ditions in vegetative cells and heterocysts. Differences between the t wo nif clusters suggest that the nitrogenase that is expressed in hete rocysts is developmentally regulated while the other is regulated by e nvironmental factors.