SEQUENCES OF NIFX, NIFW, NIFZ, NIFB AND 2 ORF IN THE FRANKIA NITROGEN-FIXATION GENE-CLUSTER

The actinomycete Frankia alni fixes N-2, in root nodules of several no n-leguminous plants. It is one of the few known N-2-fixing members of the high-GC Gram(+) lineage of prokaryotes. Thus, we have undertaken a study of its nitrogen fixation gene (nif) organization to compare wit h that of the more extensively characterized proteobacteria. A cosmid (pFN1) containing the nif region of Fa CpI1 was isolated from a cosmid library using the nifHDK genes of Fa CpI1 as a probe. A 4.5-kb BamHI fragment that mapped downstream from the previously characterized nifH DK genes was cloned and sequenced. Based on nt and aa sequence similar ities to nif from other N-2-fixing bacteria, eight ORF were identified and designated nifX, orf3, orf1, nifW, nifZ, nifB, orf2 and nifU. A r egion that hybridized to Rhizobium meliloti and Klebsiella pneumoniae nifA did not appear to contain a nifA-like gene. We have revised the m ap of the Fa nif region to reflect current information.