3 '-End processing of pre-mRNA in eukaryotes

3'-Ends of almost all eukaryotic mRNAs are generated by endonucleolytic cle avage and addition of a poly(A) tail. In mammalian cells, the reaction depe nds on the sequence AAUAAA upstream of the cleavage site, a degenerate GU-r ich sequence element downstream of the cleavage site and stimulatory sequen ces upstream of AAUAAA, Six factors have been identified that carry out the two reactions, With a single exception, they have been purified to homogen eity and cDNAs for 11 subunits have been cloned. Some of the cooperative RN A-protein and protein-protein interactions within the processing complex ha ve been analyzed, but many details, including the identity of the endonucle ase, remain unknown. Several examples of regulated polyadenylation are bein g analyzed at the molecular level. In the yeast Saccharomyces cerevisiae, s equences directing cleavage and polyadenylation are more degenerate than in metazoans, and a downstream element has not been identified. The list of p rocessing factors may be complete now with approximately a dozen polypeptid es, but their functions in the reaction are largely unknown. 3'-Processing is known to be coupled to transcription. This connection is thought to invo lve interactions of processing factors with the mRNA cap as well as with RN A polymerase II. (C) 1999 Published by Elsevier Science B.V. All rights res erved.