The Rhizobium leucaenae CFN 299 pSym plasmid contains genes expressed in free life and symbiosis, as well as two replication systems

Abstract

The alpha-protobacterium Rhizobium leucaenae CFN 299 is able to nodulate and fix nitrogen in symbiosis with a wide range of legumes, including Phaseolus vulgaris (common bean). Strain CFN 299 contains a 500-kb symbiotic plasmid (pSym) that encodes genes required for nodulation and nitrogen fixation as well as many genes whose function is unknown. In this work, we characterized the transcriptional expression of 16 pSym genes in common bean nodules and in free-living cells grown in culture. A functionally diverse group of genes were expressed during discrete stages of the symbiosis or in free-living cells. These included genes whose products are involved in nodulation and nitrogen fixation, carbon metabolism, vitamin synthesis, sulfur utilization, conjugation, transposition and DNA replication. We also examined the functionality of two replication systems encoded on pSym and found that repABC, but not repC2, is required for pSym replication.