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Summary: We have previously reported that Rhizobium phaseoli has multiple copies of nitrogen-fixation gene sequences. In this work we extend our analysis to cover a broader range of R. phaseoli strains and other rhizobia isolated from different legumes. Our results indicate that most R. phaseoli strains have reiterated nifH sequences. Reiterations are also found in rhizobia from nodules of other Phaseolus species and from the close relative of Phaseolus, Pachyrhizus. However, nifH gene reiteration is not always found in the rhizobia able to nodulate and fix nitrogen in Phaseolus, since some strains isolated from nodules of Phaseolus vulgaris as well as from other legumes are able to establish an effective symbiosis with Phaseolus vulgaris and do not show reiterations. We propose that there are different evolutionary lines of R. phaseoli. The reiteration of nif genes may be considered a marker for the most abundant and specialized of these evolutionary lines.
Martínez‐Romero et al. (Mon,) studied this question.