AT-AC Introns—An ATtACk on Dogma

Messenger RNAs are initially made as long precursor molecules from which final, transcribable mRNAs are assembled. A specialized organelle, the spliceosome, contains small RNAs with pairs of dinucleotides that can pick out complementary pairs of nucleotides at the ends of the introns that are to be spliced together. It has been assumed that these pairs were always GU-AG (or GT-AG in the DNA), but in findings described in this week's issue of Science (p. 1716) and the current issue of Cell (vol. 84, p. 801) a minor class of intron is shown to be flanked by AU-AC (or AT-AC in the DNA). In his Perspective, Mount explains how these maverick introns were discovered and why their existence is so surprising.