AT-AC Introns: An ATtACk on Dogma

摘要

In the years immediately after the discovery that messenger RNAs (mRNAs) were spliced together from larger precursors, a common sequence feature was described in these precursors--invariant GT and AG dinucleotides at the ends of the in-tron (corresponding to GU and AG in die RNA). These short, conserved sequences (see figure) reflect the mechanism by which introns are removed from the precursor, and are not found in self-splicing, organelle, or transfer RNA introns (which are removed by other' mechanisms). -Among pre-mRHA introns, exceptibns to this consensus ate rare, and greater than 99% of pre-mRNA splice sites conform to the consensus sequences in the figure (1). The extent of agreement varies, but the "GT-AG" rule is followed particularly well.Apparent exceptions that prove die rule include 5' splice sites with C at die second position and an otherwise excellent match to consensus. Most exceptions in the databases can be attributed to sequencing'errors, genetic polymorphisms, somatic mutation, or errors in database annotation (1).