The Polyadenylation of RNA in Plants

Poly(A) tracts are a nearly ubiquitous feature of mRNAs in eukaryotes. The poly(A) tail is an important determinant of the function of a eukaryotic mRNA, because it is intimately involved in processes that determine the translatability (Sachs and Wahle, 1993) and lifetime (Jacobson and Peltz, 1996) of mRNAs. From this perspective, the addition of the poly(A) tail to an mRNA is an important, even necessary, step in the expression of eukaryotic genes. However, although the functions of a poly(A) tail are largely manifest in the cytoplasm, the process of polyadenylation itself is an important component of mRNA metabolism. A growing body of evidence indicates that mRNA polyadenylation may be physically linked to the processes of intron remova1 (e.g. Niwa et al., 1990) and transcription termination (McCracken et al., 1997), and other studies imply an interplay between polyadenylation and transport of mRNA (Huang and Carmichael, 1996). Thus, it may be more appropriate to consider mRNA 3'-end formation as part of a larger series of events that begins with the initiation of transcription by RNA polymerase I1 and ends with the delivery of a mature, polyadenylated mRNA to the cytoplasm. In this context, the individual components that mediate mRNA polyadenylation may be expected to have an impact on other nuclear processes (splicing, trankription termination, and transport) as well. Several recent reviews may be consulted for a detailed picture of the process by which mRNAs are polyadenylated in animals and yeast (Wahle, 1995; Proudfoot, 1996; Wahle and Keller, 1996). Briefly, poly(A) tails are nontemplated and are thus added to nuclear mRNA precursors in a posttranscriptional process. Moreover, polyadenylation in the nucleus is usually viewed as an RNA processing event; in other words, the 3' end of the mRNA to which poly(A) i s added is generated, not by termination of transcription by RNA polymerase 11, but rather by processing of a larger RNA that is generated by transcription beyond the poly(A) site. In mammals and yeast a finite series of factors (5-6) cooperate to recognize, process, and polyadenylate precursor mRNAs in the nucleus. Several of the

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