Mechanism and regulation of mRNA polyadenylation.

A poly(A) tail is found at the 38 end of nearly every fully processed eukaryotic mRNA and has been suggested to influence virtually all aspects of mRNA metabolism. Its proposed functions include conferring mRNA stability, promoting an mRNA’s translational efficiency, and having a role in transport of processed mRNA from the nucleus to the cytoplasm (for recent reviews, see Lewis et al. 1995; Sachs et al. 1997; Wickens et al. 1997). The reaction that catalyzes the addition of the poly(A) tail, an endonucleolytic cleavage followed by poly(A) synthesis, has also been the focus of intense investigation but, until recently, may have been viewed as a process that follows a predictable, isolated, and invariant path. Yet, as more is learned about 38-end formation, it becomes clear that the function of the polyadenylation machinery extends beyond simply adding poly(A) tails to mRNAs. The first report of a component of the mammalian cleavage and polyadenylation machinery was nearly 40 years ago in a paper describing an activity found in thymus nuclei extracts that could synthesize poly(A) from ATP (Edmonds and Abrams 1960). Ten years passed before poly(A) tails were identified as a post-transcriptionally added modification of mRNA 38 termini and a possible function was assigned to poly(A) polymerase (Darnell et al. 1971; Edmonds et al. 1971; Lee et al. 1971). But nearly another decade elapsed before it was found that transcription proceeds past the polyadenylation site, revealing that a mechanism other than transcriptional termination generates mRNA 38 ends (Ford and Hsu 1978; Nevins and Darnell 1978; Manley et al. 1982). The pace of discovery quickened with the development of cell extracts that reproduce the reaction, and this allowed the subsequent and still ongoing biochemical characterization of mRNA 38-end formation (Manley 1983; Moore and Sharp 1984, 1985). The results from this work have shown that nuclear cleavage and poly(A) addition occurs in a coupled reaction and is carried out by a suprisingly large complex of multisubunit proteins (for recent reviews, see Keller 1995; Manley 1995). Several years were devoted to detailing the mechanism of 38-end formation, assigning relatively simple functions to each separable factor of the complex polyadenylation machinery. With the cloning of cDNAs encoding many of these factors, we have enjoyed an accelerated pace in understanding their precise functions, as well as the unexpected bonuses of finding that these basal factors link nuclear polyadenylation to a variety of cellular processes and that they can be important targets for regulating gene expression. Here we describe how information gained from studies done over the last few years has enhanced our understanding of the structure and function of the proteins catalyzing polyadenylation. We concentrate on mammalian systems but also highlight progress that points to both similarities and differences in yeast polyadenylation. From reviewing the latest events, we not only see how far we have come in 40 years but also become more aware of the rich path of discovery that lies ahead.