CHARACTERIZATION OF NIFB, NIFS, AND NIFU GENES IN THE CYANOBACTERIUM ANABAENA-VARIABILIS - NIFB IS REQUIRED FOR THE VANADIUM-DEPENDENT NITROGENASE

Anabaena variabilis ATCC 29413 is a heterotrophic, nitrogen-fixing cya nobacterium containing both a Mo-dependent nitrogenase encoded by the nif genes and V-dependent nitrogenase encoded by the vnf genes. The ni fB, nifS, and nifU genes of A. variabilis were cloned, mapped, and par tially sequenced. The fdxN gene was between nifB and nifS. Growth and acetylene reduction assays using mild-type and mutant strains indicate d that the nifB product (NifB) was required for nitrogen fixation not only-by the enzyme encoded by the nif genes but also by the enzyme enc oded by the vnf genes. Neither NifS nor NifU was essential for nitroge n fixation in A. variabilis.