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

An approximately 16-kb fragment of the Trichodesmium sp. IMS101 (a nonheterocystous filamentous cyanobacterium) conventional nif gene cluster was cloned 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 frames (ORF3 and ORF1) in Trichodesmium. The Trichodesmium nifEN genes encode a fused NifEN polypeptide that does not appear to be processed into individual NifE and NifN polypeptides. Fused nifEN genes were previously found in the A. variabilis nif2 genes, but we have found that fused nifEN genes are widespread in the nonheterocystous cyanobacteria. Although the gene organization 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 Trichodesmium and A. variabilis vegetative (nif2) nitrogenase genes.