Deletion of a conserved dinucleotide inhibits the second step of group II intron splicing

Few point mutations have been described that specifically inhibit the second step of group II intron splicing. Furthermore, the effects of such mutations are typically not apparent unless the mutations are studied in the context of a substrate that harbors a very short 5' exon. Truncation of the 5' exon slows the second step of splicing. Once the second step has been slowed, the effects of point mutations can be seen. We report the unexpected observation that the deletion of a conserved GA dinucleotide dramatically inhibits the second step of splicing, even when the mutation is studied in the context of a full-length substrate. In contrast, we find that this mutation does not significantly affect the first step of splicing, unless the mutation is studied in combination with a second point mutation that is known to inhibit the first step. Even in that context, the effect of the GA deletion mutation on the first step is modest. These observations, together with the inferred location of the GA dinucleotide in the three-dimensional structure of the intron, suggest that this dinucleotide plays a particularly important role in the second step of splicing.