Spliceosomal introns in conserved sequences of U1 and U5 small nuclear RNA genes in yeast Rhodotorula hasegawae.

U1, U2, U4, U5, and U6 small nuclear RNAs are essential for precursor mRNA splicing. We previously found one and four mRNA-type, or spliceosomal introns, in the U2 and U6 genes, respectively, of yeast Rhodotorula hasegawae (Erythrobasidium hasegawianum). We proposed that U2 and U6 RNAs form a catalytic core for precursor mRNA splicing and that the introns in those small nuclear RNA genes may have been acquired through reverse splicing of an intron from a precursor mRNA into a catalytic site in small nuclear RNAs. In the present study, we analyzed U1, U4, and U5 genes in R. hasegawae. One spliceosomal intron was found in the U1 region forming base-pairs with a 5' splice site of a precursor mRNA. The U5 gene has two spliceosomal introns in the region that interacts with 5' and 3' splice sites. In contrast, the gene for U4 RNA, which is released from the spliceosome prior to the first step of the splicing reaction, has no intron. These results lend a further support to the proposed relation between presence and position of an intron in an small nuclear RNA gene and the function of the encoded small nuclear RNA.