2 GROUP-I INTRONS WITH A C-CENTER-DOT-G BASEPAIR AT THE 5'-SPLICE-SITE INSTEAD OF THE VERY HIGHLY CONSERVED U-CENTER-DOT-G BASEPAIR - IS SELECTION POSTTRANSLATIONAL

In virtually all of the 200 group I introns sequenced thus far, the sp ecificity of 5' splice-site cleavage is determined by a basepair betwe en 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-si te, It has been reported that two introns in the cytochrome oxidase su bunit I gene of Aspergillus nidulans and Podospora anserina are except ions 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 editi ng does not convert the C to a U, Both introns autocatalytically cleav e 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.