Boundaries for Conjugative Transfer of Rhizobial Plasmids: Restraining and Releasing Factors

Abstract

Rhizobia are a very interesting group of bacteria, able to differentiate into nitrogen-fixing bacteroids inside nodules, formed through their symbiotic interaction with the roots of leguminous plants. Most of the bacterial genetic information required for the establishment of the symbiosis and nitrogen fixation is usually localized on plasmids, called symbiotic (pSym). In addition to a pSym, the genomes of rhizobia may carry up to 60 % of their sequences in a variable number of plasmids. In general, plasmids are considered elements likely to perform horizontal gene transfer through conjugation. Analysis of the conjugation ability of rhizobial plasmids has revealed the participation of an assortment of regulatory elements, including quorum-sensing, RctA-mediated repression, and other recently described modulators. In addition to which regulators are present in a strain, their genomic localization is also a significant factor influencing the scope of plasmid transfer. Integrating the available data, we propose that plasmid transfer is delimited by a complex set of intra- and inter-replicon interactions, which respond to specific environmental signals.