The tRNAargGene andengAAre Essential Genes on the 1.7-Mb pSymB Megaplasmid of Sinorhizobium meliloti and Were Translocated Together from the Chromosome in an Ancestral Strain Academic Article uri icon

  •  
  • Overview
  •  
  • Research
  •  
  • Identity
  •  
  • Additional Document Info
  •  
  • View All
  •  

abstract

  • ABSTRACTBacterial 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 meliloti1021 contains a 3.7-Mb chromosome and 1.4-Mb (pSymA) and 1.7-Mb (pSymB) megaplasmids. In this study, the tRNAargandengAgenes, 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 frediiNGR234, the tRNAargandengAgenes are located on the chromosome, in a 69-kb region designated theengA-tRNAarg-rmlCregion. This region includesbacA, a gene that is important for intracellular survival during host-bacterium interactions forS. melilotiand the related alphaproteobacteriumBrucella abortus. TheengA-tRNAarg-rmlCregion lies between thekdgKanddppF2(NGR_c24410) genes on theS. frediichromosome. Synteny analysis showed thatkdgKanddppF2orthologues are adjacent to each other on the chromosomes of 15 sequenced strains ofS. melilotiandSinorhizobium medicae, whereas the 69-kbengA-tRNAarg-rmlCregion is present on the pSymB-equivalent megaplasmids. This and other evidence strongly suggests that theengA-tRNAarg-rmlCregion translocated from the chromosome to the progenitor of pSymB in an ancestor common toS. melilotiandS. medicae. To our knowledge, this work represents one of the first experimental demonstrations that essential genes are present on a megaplasmid.

publication date

  • January 15, 2013