RETROHOMING OF A BACTERIAL GROUP-II INTRON - MOBILITY VIA COMPLETE REVERSE SPLICING, INDEPENDENT OF HOMOLOGOUS DNA RECOMBINATION

The mobile group II intron of Lactococcus lactis, LI.LtrB, provides th e opportunity to analyze the homing pathway in genetically tractable b acterial systems. Here, we show that LI.LtrB mobility occurs by an RNA -based retrohoming mechanism in both Escherichia coli and L. lactis. S urprisingly, retrohoming occurs efficiently in the absence of RecA fun ction, with a relaxed requirement for flanking exon homology and witho ut coconversion of exon markers. These results lead to a model for bac terial retrohoming in which the intron integrates into recipient DNA b y complete reverse splicing and serves as the template for cDNA synthe sis. The retrohoming reaction is completed in unprecedented fashion by a DNA repair event that is independent of homologous recombination be tween the alleles. Thus, LI.LtrB has many features of retrotransposons , with practical and evolutionary implications.