title: Circularization pathway of a bacterial group II intron.
abstract: Group II introns are large RNA enzymes that can excise as lariats, circles or in a linear form through branching, circularization or hydrolysis, respectively. Branching is by far the main and most studied splicing pathway while circularization was mostly overlooked. We previously showed that removal of the branch point A residue from Ll.LtrB, the group II intron from Lactococcus lactis, exclusively leads to circularization. However, the majority of the released intron circles harbored an additional C residue of unknown origin at the splice junction. Here, we exploited the Ll.LtrB-ΔA mutant to study the circularization pathway of bacterial group II introns in vivo. We demonstrated that the non-encoded C residue, present at the intron circle splice junction, corresponds to the first nt of exon 2. Intron circularization intermediates, harboring the first 2 or 3 nts of exon 2, were found to accumulate showing that branch point removal leads to 3' splice site misrecognition. Traces of properly ligated exons were also detected functionally confirming that a small proportion of Ll.LtrB-ΔA circularizes accurately. Overall, our data provide the first detailed molecular analysis of the group II intron circularization pathway and suggests that circularization is a conserved splicing pathway in bacteria.
authors.affiliation: Department of Microbiology and Immunology, Microbiome and Disease Tolerance Centre (MDTC), McGill University, Montréal, Québec, Canada H3A 2B4.
authors.forename: Caroline
authors.initials: C
authors.lastname: Monat
authors.affiliation: Department of Microbiology and Immunology, Microbiome and Disease Tolerance Centre (MDTC), McGill University, Montréal, Québec, Canada H3A 2B4 benoit.cousineau@mcgill.ca.
authors.forename: Benoit
authors.initials: B
authors.lastname: Cousineau
mesh_terms: D004251:DNA Transposable Elements; D005091:Exons / Q000235:genetics; D007438:Introns / Q000235:genetics*; D013294:Lactococcus lactis / Q000235:genetics*; D053858:Metabolic Networks and Pathways / Q000235:genetics; D012313:RNA / Q000235:genetics*; D012326:RNA Splicing / Q000235:genetics*
chemical_list: D004251:DNA Transposable Elements; D012313:RNA