PHYLOGENY OF CYANOBACTERIAL NIFH GENES - EVOLUTIONARY IMPLICATIONS AND POTENTIAL APPLICATIONS TO NATURAL ASSEMBLAGES

DNA sequences of a fragment of nifH from diverse cyanobacteria were am plified, cloned and sequenced to determine the evolutionary relationsh ip of nitrogenase within the cyanobacteria as a group, and to provide a basis for the identification of uncultivated strains of cyanobacteri a in the environment. Analysis of 30 nitrogenase DNA and deduced amino acid sequences from cyanobacteria representing five major taxonomic s ubdivisions showed great variation in phylogenetic distances between t he sequences. Sequences from heterocystous cyanobacteria formed a cohe rent cluster, in which branching forms did not form a clade distinct f rom the non-branching forms. Nitrogenase sequences from the unicellula r cyanobacteria Gloeothece and Synechococcus sp. RF-1 formed a cluster , as did sequences from the genera Xenococcus and Myxosarcina. The nif H sequences of filamentous nonheterocystous cyanobacteria were not clo sely related to each other, forming deep branches with respect to the heterocystous cyanobacterial nifH sequences. The phylogeny of nifH bas ed on amino acid sequences was consistent with taxonomic relationships among the strains; for example, a sequence obtained from a natural as semblage believed to be dominated by 'Lyngbya' clustered with nifH fro m Lyngbya lagerheimii. Results also indicate that the phylogeny of nif H among the cyanobacteria is largely consistent with the phylogeny of 16S rRNA, and furthermore that the nifH sequence can be used to identi fy uncultivated strains of nitrogen-fixing cyanobacteria.