The mobile group II intron of Lacfococcus lactis, Ll.LtrB, provides the opportunity to analyze the homing pathway in genetically tractable bacterial systems. Here, we show that Ll.LtrB mobility occurs by an RNA-based retrohoming mechanism in both Escherichia coil 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, Ll.LtrB has many features of retrotransposons, with practical and evolutionary implications.
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We thank M. Belisle for figures, M. Carl for preparing the MS, and M. Cusick, V. Derbyshire, R. Lease, and M. Parker for useful comments. This work was supported by National Institutes of Health grants GM39422 and GM44844 to M. B. and GM37949 to A. M. L.; B. C. was supported by a fellowship from FCAR (Fonds pour la Formation de Chercheurs et l’Aide à la Recherche, Québec).