Cloning Defined Regions of the pExo Megaplasmid of Rhizobium (Sinorhizobium) Meliloti

Bacteria that form nitrogen-fixing nodules on the roots of plants have been the subject of much research over the past twenty years. This has led to major breakthroughs in understanding how bacteria interact with plants and fix nitrogen. The model system most extensively studied is the interaction between the bacterium Rhizobium meliloti and alfalfa. The R. meliloti genome consists of three replicons, a chromosome of 3540 Kb and two megaplasmids, pSym (pRmeSU47a) and pExo (pRmeSU47b), of 1340 and 1700 Kb respectively (Honeycutt et al., 1993). Each of these plasmids is larger in overall size than the genomes of some bacteria, such as Mycoplasma genitalium (580 Kb) (Fraser et al., 1995). They contain many genes involved in the formation of nitrogen-fixing nodules, as well as genes important for survival and, presumably, competitiveness of the organism within the soil. The nature of these genes is of major current interest.