title: Two group I introns with a C.G basepair at the 5' splice-site instead of the very highly conserved U.G basepair: is selection post-translational?
abstract: In virtually all of the 200 group I introns sequenced thus far, the specificity of 5' splice-site cleavage is determined by a basepair between a uracil base at the end of the 5' exon and a guanine in an intron guide sequence which pairs with the nucleotides flanking the splice-site. It has been reported that two introns in the cytochrome oxidase subunit I gene of Aspergillus nidulans and Podospora anserina are exceptions to this rule and have a C.G basepair in this position. We have confirmed the initial reports and shown for one of them that RNA editing does not convert the C to a U. Both introns autocatalytically cleave the 5' splice-site. Mutation of the C to U in one intron reduces the requirement for Mg2+ and leads to an increase in the rate of cleavage. As the C base encodes a highly conserved amino acid, we propose that it is selected post-translationally at the level of protein function, despite its inferior splicing activity.
authors.affiliation: Department of Biology, Temple University, Philadelphia, PA 19122, USA.
authors.forename: M
authors.initials: M
authors.lastname: Hur
authors.forename: R B
authors.initials: RB
authors.lastname: Waring
mesh_terms: D001203:Ascomycota / Q000235:genetics; D001233:Aspergillus nidulans / Q000235:genetics; D001482:Base Composition; D001483:Base Sequence; D003001:Cloning, Molecular; D017124:Conserved Sequence; D003562:Cytidine / Q000235:genetics; D017931:DNA Primers; D003576:Electron Transport Complex IV / Q000235:genetics*; D005658:Fungi / Q000235:genetics*; D007438:Introns / Q000235:genetics*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D016133:Polymerase Chain Reaction; D011499:Protein Processing, Post-Translational; D012326:RNA Splicing; D012641:Selection, Genetic; D014529:Uridine / Q000235:genetics
chemical_list: D017931:DNA Primers; D003562:Cytidine; D003576:Electron Transport Complex IV; D014529:Uridine