Rhizobium phaseoli: A molecular genetics view

Abstract

We have used molecular genetics techniques to analyze the structural and functional organization of genetic information ofRhizobium phaseoli, the symbiont of the common bean plantPhaseolus vulgaris. As in otherRhizobium species, the genome consists of the chromosome and plasmids of high molecular weight. Symbiotic determinants, nitrogen fixation genes as well as nodulation genes, are localized on a single replicon, the symbiotic (sym) plasmid. Thesym plasmid of differentR. phaseoli strains was transferred to anAgrobacterium tumefaciens strain cured of its native plasmids. In all cases, Agrobacterium transconjugants able to nodulate bean plants were obtained. Some of the transconjugants had the capacity to elicit an effective symbiosis. The genome ofR. phaseoli is complex, containing a large amount of reiterated DNA sequences. In mostR. pahseoli strains one of such reiterated DNA families corresponds to the nitrogenase structural genes (nif genes). A functional analysis of these genes suggested that the presence of reiteratednif genesis is related to the capacity of fixing atmospheric nitrogen in the symbiotic state. The presence of several repeated sequences in the genome might provide sites for recombination, resulting in genomic rearrangements. By analyzing direct descendants of a single cell in the laboratory, evidence of frequent genomic rearrangements inR. phaseoli was found. We propose that genomic rearrangements constitute the molecular basis of the frequent variability and loss of symbiotic properties in different Rhizobium strains.