Complete genome structure of the nitrogen-fixing symbiotic bacterium Mesorhizobium loti

The complete nucleotide sequence of the genome of a symbiotic bacterium Mes orhizobium loti strain MAFF303099 was determined. The genome of M. loti con sisted of a single chromosome (7,036, 071 bp) and two plasmids, designated as pMLa (351, 911 bp) and pMLb (208, 315 bp). The chromosome comprises 6752 potential protein-coding genes, two sets of rRNA genes and 50 tRNA genes r epresenting 47 tRNA species. Fifty-four percent of the potential protein ge nes showed sequence similarity to genes of known function, 21% to hypotheti cal genes, and the remaining 25% had no apparent similarity to reported gen es. A 611-kb DNA segment, a highly probable candidate of a symbiotic island , was identified, and 30 genes for nitrogen fixation and 24 genes for nodul ation were assigned in this region. Codon usage analysis suggested that the symbiotic island as well as the plasmids originated and were transmitted f rom other genetic systems. The genomes of two plasmids, pMLa and pMLb, cont ained 320 and 209 potential protein-coding genes, respectively, for a varie ty of biological functions. These include genes for the ABC-transporter sys tem, phosphate assimilation, two-component system, DNA replication and conj ugation, but only one gene for nodulation was identified.