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Molecular evolution of the nif gene cluster carrying nifI₁ and nifI₂ genes in the Gram-positive phototrophic bacterium Heliobacterium chlorum

The International Center for Biotechnology, Osaka University, 2-1 Yamada-oka, Suita-shi, Osaka 565-0871, Japan

Correspondence
Hiroko Kawasaki
ICBKawasakiNakagawa{at}icb.osaka-u.ac.jp

A major nif cluster was detected in the strictly anaerobic, Gram-positive phototrophic bacterium Heliobacterium chlorum. The cluster consisted of 11 genes arranged within a 10 kb region in the order nifI₁, nifI₂, nifH, nifD, nifK, nifE, nifN, nifX, fdx, nifB and nifV. The phylogenetic position of Hbt. chlorum was the same in the NifH, NifD, NifK, NifE and NifN trees; Hbt. chlorum formed a cluster with Desulfitobacterium hafniense, the closest neighbour of heliobacteria based on the 16S rRNA phylogeny, and two species of the genus Geobacter belonging to the Deltaproteobacteria. Two nifI genes, known to occur in the nif clusters of methanogenic archaea between nifH and nifD, were found upstream of the nifH gene of Hbt. chlorum. The organization of the nif operon and the phylogeny of individual and concatenated gene products showed that the Hbt. chlorum nif operon carrying nifI genes upstream of the nifH gene was an intermediate between the nif operon with nifI downstream of nifH (group II and III of the nitrogenase classification) and the nif operon lacking nifI (group I). Thus, the phylogenetic position of Hbt. chlorum nitrogenase may reflect an evolutionary stage of a divergence of the two nitrogenase groups, with group I consisting of the aerobic diazotrophs and group II consisting of strictly anaerobic prokaryotes.

The GenBank/EMBL/DDBJ accession number for the complete coding sequence of the nif gene cluster of Hbt. chlorum DSM 3682^T is AB196525.

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