Bacterial group II introns generate genetic diversity by circularization and trans-splicing from a population of intron-invaded mRNAs.

Group II introns are ancient retroelements that significantly shaped the origin and evolution of contemporary eukaryotic genomes. These self-splicing ribozymes share a common ancestor with the telomerase enzyme, the spliceosome machinery as well as the highly abundant spliceosomal introns and non-LTR retroelements. More than half of the human genome thus consists of various elements that evolved from ancient group II introns, which altogether significantly contribute to key functions and genetic diversity in eukaryotes. Similarly, group II intron-related elements in bacteria such as abortive phage infection (Abi) retroelements, diversity generating retroelements (DGRs) and some CRISPR-Cas systems have evolved to confer important functions to their hosts. In sharp contrast, since bacterial group II introns are scarce, irregularly distributed and frequently spread by lateral transfer, they have mainly been considered as selfish retromobile elements with no beneficial function to their host. Here we unveil a new group II intron function that generates genetic diversity at the RNA level in bacterial cells. We demonstrate that Ll.LtrB, the model group II intron from Lactococcus lactis, recognizes specific sequence motifs within cellular mRNAs by base pairing, and invades them by reverse splicing. Subsequent splicing of ectopically inserted Ll.LtrB, through circularization, induces a novel trans-splicing pathway that generates exon 1-mRNA and mRNA-mRNA intergenic chimeras. Our data also show that recognition of upstream alternative circularization sites on intron-interrupted mRNAs release Ll.LtrB circles harboring mRNA fragments of various lengths at their splice junction. Intergenic trans-splicing and alternative circularization both produce novel group II intron splicing products with potential new functions. Overall, this work describes new splicing pathways in bacteria that generate, similarly to the spliceosome in eukaryotes, genetic diversity at the RNA level while providing additional functional and evolutionary links between group II introns, spliceosomal introns and the spliceosome.