Two group I introns with a OG basepair at the 5′splice-site instead of the very highly conserved LUG 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 Asperglllus nldulans 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. © 1995 Oxford University Press.