PRE-MESSENGER-RNA TOPOLOGY IS IMPORTANT FOR 3'-END FORMATION IN SACCHAROMYCES-CEREVISIAE AND MAMMALS

Various signal motifs that are required for efficient pre-mRNA 3'-end formation in the yeast Saccharomyces cerevisiae have been reported, No ne of these known signal sequences appears to be of the same general i mportance as is the mammalian AAUAAA motif, To establish the importanc e of yeast pre-mRNA termini in 3'-end formation, the ends of a pre-mRN A transcript synthesized in vitro were ligated before incubation in a yeast whole-cell extract, Such covalently closed circular RNAs were no t cleaved at their poly(A) sites. Interestingly, pseudocircular RNAs w ith complementary 3'- and 5'-terminal sequences allowing the formation of panhandle structures were also resistant to cleavage, However, 3'- end processing was impeded neither by terminal hairpins at either or a t both ends nor by RNA oligonucleotides complementary to either or bot h ends of a linear pre-mRNA, Intriguingly, mammalian pseudocircular pr e-mRNAs also were not cleaved at their poly(A) sites when incubated in a HeLa cell nuclear extract, These results provide evidence for the g eneral importance of RNA topology in the formation of an active 3'-end processing complex in S. cerevisiae and higher eukaryotes. The possib ility of a torus-shaped factor involved in 3'-end formation is discuss ed.