Sporadic Distribution of tRNA(CCU)(Arg) Introns among α-Purple Bacteria: Evidence for Horizontal Transmission and Transposition of a Group I Intron

A group I intron interrupts the tRNA(CCU)(Arg) gene of the α-purple bacterium Agrobacterium tumefaciens (B. Reinhold-Hurek and D. A. Shub, Nature [London] 357:173–176, 1992). In this study, we assess the distribution of the corresponding intron among 12 additional species of α-purple bacteria. Of 10 newly identified tRNA(CCU)(Arg) genes, we found only two that contained an intron homologous to that of the Agrobacterium tRNA(CCU)(Arg) intron. This restricted and scattered distribution of the tRNA(CCU)(Arg) intron among α-purple bacteria is consistent with a recent origin and horizontal transmission. Primary and secondary structural similarities between tRNA(UAA)(Leu) introns found in strains of the cyanobacterium Microcystis aeruginosa (K. Rudi and K. S. Jacobsen, FEMS Microbiol. Lett. 156:293–298, 1997) and α-purple tRNA(CCU)(Arg) introns suggest that these introns share a more recent common ancestor than either does with other known cyanobacterial tRNA(UAA)(Leu) introns.