RNA splicing and intron turnover are greatly diminished by a mutant yeast branch point

Two mutant genes, both of which contain an A----C transversion at the absolutely conserved branch point of the yeast "TACTAAC box" (TACTAAC----TACTACC), were constructed and introduced into yeast cells. Splicing and gene expression are almost completely eliminated by this mutation, but a low level (approximately equal to 0.1%) of proper splicing is detectable. Branch point mapping indicates that the mutant branch is formed at the normal location--i.e., to cytidine rather than adenosine. The mutant branch is also a very poor substrate for the HeLa cell debranching enzyme. Although splicing of the mutant transcripts is very poor, the cells contain a high level of mutant intron because these excised introns are remarkably stable. The results imply that the normal branch point is important not only for branch formation and splicing but also for intron turnover.