title: The Ll.LtrB intron from Lactococcus lactis excises as circles in vivo: insights into the group II intron circularization pathway.
abstract: Group II introns are large ribozymes that require the assistance of intron-encoded or free-standing maturases to splice from their pre-mRNAs in vivo. They mainly splice through the classical branching pathway, being released as RNA lariats. However, group II introns can also splice through secondary pathways like hydrolysis and circularization leading to the release of linear and circular introns, respectively. Here, we assessed in vivo splicing of various constructs of the Ll.LtrB group II intron from the Gram-positive bacterium Lactococcus lactis. The study of excised intron junctions revealed, in addition to branched intron lariats, the presence of perfect end-to-end intron circles and alternatively circularized introns. Removal of the branch point A residue prevented Ll.LtrB excision through the branching pathway but did not hinder intron circle formation. Complete intron RNA circles were found associated with the intron-encoded protein LtrA forming nevertheless inactive RNPs. Traces of double-stranded head-to-tail intron DNA junctions were also detected in L. lactis RNA and nucleic acid extracts. Some intron circles and alternatively circularized introns harbored variable number of non-encoded nucleotides at their splice junction. The presence of mRNA fragments at the splice junction of some intron RNA circles provides insights into the group II intron circularization 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.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Quiroga
authors.affiliation: Department of Microbiology and Immunology, Microbiome and Disease Tolerance Centre (MDTC), McGill University, Montréal, Québec, Canada H3A 2B4.
authors.forename: Felix
authors.initials: F
authors.lastname: Laroche-Johnston
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: D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D005798:Genes, Bacterial; D047468:Immunoprecipitation; D007438:Introns; D013294:Lactococcus lactis / Q000235:genetics*; D016133:Polymerase Chain Reaction; D016337:RNA, Catalytic / Q000378:metabolism
chemical_list: D004269:DNA, Bacterial; D016337:RNA, Catalytic; C418481:hairpin ribozyme
keywords: alternative splicing; circularization pathway; group II intron; head-to-tail intron DNA junction; intron circle