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 the opportunity to analyze the homing pathway in genetically tractable bacterial systems. Here, we show that LI.LtrB mobility occurs by an RNA-based retrohoming mechanism in both Escherichia coli and L. lactis. Surprisingly, retrohoming occurs efficiently in the absence of RecA function, with a relaxed requirement for flanking exon homology and without coconversion of exon markers. These results lead to a model for bacterial retrohoming in which the intron integrates into recipient DNA by complete reverse splicing and serves as the template for cDNA synthesis. The retrohoming reaction is completed in unprecedented fashion by a DNA repair event that is independent of homologous recombination between the alleles. Thus, LI.LtrB has many features of retrotransposons, with practical and evolutionary implications.