The tRNA arg Gene and engA Are Essential Genes on the 1.7-Mb pSymB Megaplasmid of Sinorhizobium meliloti and Were Translocated Together from the Chromosome in an Ancestral Strain

Bacterial genomes with two (or more) chromosome-like replicons are known, and these appear to be particularly frequent in alphaproteobacteria. The genome of the N2 -fixing alfalfa symbiontSinorhizobium meliloti 1021 contains a 3.7-Mb chromosome and 1.4-Mb (pSymA) and 1.7-Mb (pSymB) megaplasmids. In this study, the tRNAarg andengA genes, located on the pSymB megaplasmid, are shown to be essential for growth. These genes could be deleted from pSymB when copies were previously integrated into the chromosome. However, in the closely related strainSinorhizobium fredii NGR234, the tRNAarg andengA genes are located on the chromosome, in a 69-kb region designated theengA- tRNAarg -rmlC region. This region includesbacA , a gene that is important for intracellular survival during host-bacterium interactions forS. meliloti and the related alphaproteobacteriumBrucella abortus . TheengA- tRNAarg -rmlC region lies between thekdgK anddppF2 (NGR_c24410) genes on theS. fredii chromosome. Synteny analysis showed thatkdgK anddppF2 orthologues are adjacent to each other on the chromosomes of 15 sequenced strains ofS. meliloti andSinorhizobium medicae , whereas the 69-kbengA- tRNAarg -rmlC region is present on the pSymB-equivalent megaplasmids. This and other evidence strongly suggests that theengA- tRNAarg -rmlC region translocated from the chromosome to the progenitor of pSymB in an ancestor common toS. meliloti andS. medicae . To our knowledge, this work represents one of the first experimental demonstrations that essential genes are present on a megaplasmid.