In vivo characterization of yeast mRNA processing intermediates.

Yeast mRNA introns contain a conserved sequence, TACTAAC, required for splicing. We previously identified a putative splicing intermediate characterized by a stop to reverse transcriptase at the TACTAAC box of the wild-type rp51A (ribosomal protein 51A gene) intron. We now show that this stop is due to a branch and occurs at the identical nucleotide in the actin intron TACTAAC box. We show further that the putative intermediate contains a complete intron and the 3' exon, but is missing the 5' exon. This RNA is largely in the form of a lariat. The lariat and the other putative splicing intermediates detected (two forms, of different molecular weights, of the excised intron and the free 5' exon) are compatible with the view that the cut at the 5' junction and lariat formation are early steps in yeast mRNA splicing and that substantial similarities exist between yeast and mammalian mRNA splicing.