The Sinorhizobium meliloti chromosomal origin of replication

Abstract

The predicted chromosomal origin of replication (oriC) from the alfalfa symbiontSinorhizobium melilotiis shown to allow autonomous replication of a normally non-replicating plasmid withinS. meliloticells. This is the first chromosomal replication origin to be experimentally localized in theRhizobiaceaeand its location, adjacent tohemE, is the same as fororiCinCaulobacter crescentus, the only experimentally characterized alphaproteobacterialoriC. Using an electrophoretic mobility shift assay and purifiedS. melilotiDnaA replication initiation protein, binding sites for DnaA were mapped in theS. meliloti oriCregion. Mutations in these sites eliminated autonomous replication.S. melilotithat expressed DnaA from a plasmidlacpromoter was observed to form pleomorphic filamentous cells, suggesting that cell division was perturbed. Interestingly, this cell phenotype is reminiscent of differentiated bacteroids found inside plant cells in alfalfa root nodules.