AN MESSENGER RNA-TYPE INTRON IS PRESENT IN THE RHODOTORULA-HASEGAWAE U2 SMALL NUCLEAR-RNA GENE

Splicing an mRNA precursor requires multiple factors involving five sm all nuclear RNA (snRNA) species called U1, U2, U4, U5, and U6. The pre sence of mRNA-type introns in the U6 snRNA genes of some yeasts led to the hypothesis that U6 snRNA may play a catalytic role in pre-mRNA sp licing and that the U6 introns occurred through reverse splicing of an intron from an mRNA precursor into a catalytic site of U6 snRNA. We c haracterized the U2 snRNA gene of the yeast Rhodotorula hasegawae, whi ch has four mRNA-type introns in the U6 snRNA gene, and found an mRNA- type intron of 60 bp. The intron of the U2 snRNA gene is present in th e highly conserved region immediately downstream of the branch site re cognition domain. Interestingly, we found that this region can form a novel base pairing with U6 snRNA. We discuss the possible implications of these findings for the mechanisms of intron acquisition and for th e role of U2 snRNA in pre-mRNA splicing.