T. Thiel et al., A 2ND NITROGENASE IN VEGETATIVE CELLS OF A HETEROCYST-FORMING CYANOBACTERIUM, Proceedings of the National Academy of Sciences of the United Statesof America, 92(20), 1995, pp. 9358-9362
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.