Splicing of the Sinorhizobium meliloti RmInt1 group II intron provides evidence of retroelement behavior

Group II introns act as both large catalytic RNAs and mobile retroelements. They are found in organelle and bacterial genomes and are spliced via a lariat intermediate, in a mechanism similar to that of spliceosomal introns. However, their distribution and insertion patterns, particularly for bacterial group II introns, suggest that they function and behave more like retroelements than organelle introns. RmInt1 is an efficient mobile intron found within the ISRm2011-2 insertion sequence in the symbiotic bacterium Sinorhizobium meliloti. This group II intron is excised, in vivo and in vitro, as intron lariats. However, the complete splicing reaction in vivo remains to be elucidated. A lacZ reporter gene system, northern blotting and real-time reverse transcription were carried out to investigate RmInt1 splicing activity. Splicing efficiency of 0.07 +/- 0.02% was recorded. These findings suggest that bacterial group II introns function more like retroelements than spliceosomal introns. Their location is consistent with a role for these introns in preventing the spread of other potentially harmful mobile elements in bacteria.