SEQUENCE SIMILARITY BETWEEN THE 73-KILODALTON PROTEIN OF MAMMALIAN CPSF AND A SUBUNIT OF YEAST POLYADENYLATION FACTOR-I

The 3' ends of most eukaryotic messenger RNAs are generated by endonuc leolytic cleavage and polyadenylation. In mammals, the cleavage and po lyadenylation specificity factor (CPSF) plays a central role in both s teps of the processing reaction. Here, the cloning of the 73-kilodalto n subunit of CPSF is reported. Sequence analyses revealed that a yeast protein (Ysh1) was highly similar to the 73-kD polypeptide. Ysh1 cons titutes a new subunit of polyadenylation factor I (PFI), which has a r ole in yeast pre-mRNA 3'-end formation. This finding was unexpected be cause in contrast to CPSF, PFI is only required for the polyadenylatio n reaction. These results contribute to the understanding of how 3'-en d processing factors may have evolved.