RHIZOBIUM PLASMIDS IN BACTERIA LEGUME INTERACTIONS

The functional analysis of plasmids in Rhizobium strains has concentra ted mainly on the symbiotic plasmid (pSym). However, genetic informati on relevant to both symbiotic and saprophytic Rhizobium life cycles, l ocalized on other 'cryptic' replicons, has also been reported. Informa tion is reviewed which concerns functional features encoded in plasmid s other than the pSym: biosynthesis of cell surface polysaccharides, m etabolic processes, the utilization of plant exudates, aromatic compou nds and diverse sugars, and features involved symbiotic performance. I n addition, factors which affect plasmid evolution through their influ ence on structural features of the plasmids, such as conjugative trans fer and genomic rearrangements, is discussed. Based on the overall dat a, we propose that together the plasmids and the chromosome constitute a fully integrated genomic complex, entailing structural features as well as saprophytic and cellular functions.