Trans-splicing versatility of the Ll.LtrB group II intron.

Group II introns are found in organelles, bacteria, and archaea. Some harbor an open reading frame (ORF) with reverse transcriptase, maturase, and occasionally endonuclease activities. Group II introns require the assistance of either intron-encoded or free-standing maturases to excise from primary RNA transcripts in vivo. Some ORF-containing group II introns were shown to be mobile retroelements that invade new DNA sites by retrohoming or retrotransposition. Group II introns are also hypothesized to be the ancestors of the spliceosome-dependent nuclear introns and the small nuclear RNAs (snRNAs--U1, U2, U4, U5, and U6) that are part of the spliceosome. The ability of some fragmented group II introns to undergo splicing in trans supports the theory that the snRNAs evolved from portions of group II introns. Here, we developed a Tn5-based genetic screen to explore the trans-splicing potential of the Ll.LtrB group II intron from the Gram-positive bacterium Lactococcus lactis. Proficient trans-splicing variants of Ll.LtrB were selected using a highly sensitive trans-splicing/conjugation screen. We report that numerous fragmentation sites located throughout Ll.LtrB support splicing in trans, showing that this intron is remarkably more tolerant to fragmentation than expected from the fragmentation sites uncovered within natural trans-splicing group II introns. This work unveils the great versatility of group II intron fragments to assemble and accurately trans-splice their flanking exons in vivo.