ASSESSING HORIZONTAL TRANSFER OF NIFHDK GENES IN EUBACTERIA - NUCLEOTIDE-SEQUENCE OF NIFK FROM FRANKIA STRAIN HFPCCI3

The structural genes for nitrogenase, nifK, nifD, and nifH, are crucia l for nitrogen fixation. Previous phylogenetic analysis of the amino a cid sequence of nifH suggested that this gene had been horizontally tr ansferred from a proteobacterium to the gram-positive/cyanobacterial c lade, although the confounding effects of paralogous comparisons made interpretation of the data difficult. An additional test of nif gene h orizontal transfer using nifD was made, but the NifD phylogeny lacked resolution. Here nif gene phylogeny is addressed with a phylogenetic a nalysis of a third and longer nif gene, nifK. As part of the study, th e nifK gene of the key taxon Frankia was sequenced. Parsimony and some distance analyses of the nifK amino acid sequences provide support fo r vertical descent of nifK, but other distance trees provide support f or the lateral transfer of the gene. Bootstrap support was found for b oth hypotheses in all trees; the nifK data do not definitively favor o ne or the other hypothesis. A parsimony analysis of NifH provides supp ort for horizontal transfer in accord with previous reports, although bootstrap analysis also shows some support for vertical descent of the orthologous nifH genes. A wider sampling of taxa and more sophisticat ed methods of phylogenetic inference are needed to understand the evol ution of nif genes. The nif genes may also be powerful phylogenetic to ols. If nifK evolved by vertical descent, it provides strong evidence that the cyanobacteria and proteobacteria are sister groups to the exc lusion of the firmicutes, whereas 16S rRNA sequences are unable to res olve the relationships of these three major eubacterial lineages.