Bradyrhizobium japonicum possesses two discrete sets of electron transfer flavoprotein genes:fixA, fixB andetfS, etfL

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Abstract
A group of four co-regulated genes (fixA, fixB, fixC, fixX) essential for symbiotic nitrogen fixation has been described in several rhizobial species, includingBradyrhizobium japonicum. The complete nucleotide sequence of theB. japonicum fixA, fixB andfixC, genes is reported here. The derived amino acid sequences confirmed the previously noted sequence similarity between FixA and the β-subunit and between FixB and the α-subunit of mammalian andParacoccus denitrificans electron transfer flavoproteins (ETF). Since the classical role of ETF is in β-oxidation of fatty acids, a process unrelated to nitrogen fixation, we rationalized thatB. japonicum ought to contain bona fideetf genes in addition to theetf-like genesfixA andfixB. Therefore, we identified, cloned, sequenced, and transcriptionally analyzed theB. japonicum etfSL genes encoding the β-and α-subunits of ETF. TheetfSL genes, but not thefix genes, are transcribed in aerobically grown cells. An amino acid sequence comparison between all available ETFs and ETF-like proteins revealed the existence of two distinguishable subfamilies. Group I comprises housekeeping ETFs that link acyl-CoA dehydrogenase reactions with the respiratory chain, such as in the fatty acid degradation pathway.B. japonicum EtfS and EtfL clearly belong to this group. Group II contains ETF-like proteins that are synthesized only under certain specific growth conditions and receive electrons from the oxidation of specific substrates. The products of the anaerobically inducedfixA andfixB genes ofB. japonicum are members of that group.B. japonicum is the first example of an organism in which genes for proteins of both groups I and II of the ETF family have been identified.

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