title: A conserved 3' extension in unusual group II introns is important for efficient second-step splicing.
abstract: The B.c.I4 group II intron from Bacillus cereus ATCC 10987 harbors an unusual 3' extension. Here, we report the discovery of four additional group II introns with a similar 3' extension in Bacillus thuringiensis kurstaki 4D1 that splice at analogous positions 53/56 nt downstream of domain VI in vivo. Phylogenetic analyses revealed that the introns are only 47-61% identical to each other. Strikingly, they do not form a single evolutionary lineage even though they belong to the same Bacterial B class. The extension of these introns is predicted to form a conserved two-stem-loop structure. Mutational analysis in vitro showed that the smaller stem S1 is not critical for self-splicing, whereas the larger stem S2 is important for efficient exon ligation and lariat release in presence of the extension. This study clearly demonstrates that previously reported B.c.I4 is not a single example of a specialized intron, but forms a new functional class with an unusual mode that ensures proper positioning of the 3' splice site.
authors.affiliation: Laboratory for Microbial Dynamics (LaMDa), Department of Pharmaceutical Biosciences, University of Oslo, Oslo, Norway.
authors.forename: Fredrik B
authors.initials: FB
authors.lastname: Stabell
authors.forename: Nicolas J
authors.initials: NJ
authors.lastname: Tourasse
authors.forename: Anne-Brit
authors.initials: AB
authors.lastname: Kolstø
mesh_terms: D001413:Bacillus thuringiensis / Q000235:genetics*; D001483:Base Sequence; D017124:Conserved Sequence; D007438:Introns; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009690:Nucleic Acid Conformation; D012326:RNA Splicing; D016337:RNA, Catalytic / Q000737:chemistry*
chemical_list: D016337:RNA, Catalytic