Organization of the nif genes of the nonheterocystous cyanobacterium Trichodesmium sp IMS101

An approximately 16-kb fragment of the Trichodesmium sp. IMS101 (a nonheter ocystous filamentous cyanobacterium) "conventional" nif gene cluster was cl oned and sequenced. The gene organization of the Trichodesmium and Anabaena variabilis vegetative (nif2) nitrogenase gene clusters spanning the region from nifB to nifW are similar except for the absence of two open reading f rames (ORF3 and ORF1) in Trichodesmium. The Trichodesmium nifEN genes encod e a fused NifEN polypeptide that does not appear to be processed into indiv idual NifE and NifN polypeptides. Fused nifEN genes were previously found i n the A. variabilis nif2 genes, but we have found that fused nifEN genes ar e widespread in the nonheterocystous cyanobacteria. Although the gene organ ization of the nonheterocystous filamentous Trichodesmium nif gene cluster is very similar to that of the A. variabilis vegetative nif2 gene cluster, phylogenetic analysis of nif sequences do not support dose relatedness of T richodesmium and A. variabilis vegetative (nif2) nitrogenase genes.