PLANT MESSENGER-RNA 3'-END FORMATION

Our understanding of how the 3' ends of mRNAs are formed in plants is rudimentary compared to what we know about this process in other eukar yotes. The salient features of plant pre-mRNAs that signal cleavage an d polyadenylation remain obscure, and the biochemical mechanism is as yet wholly uncharacterised. Nevertheless, despite the lack of universa lly conserved cis-acting motifs, a common underlying architecture is e merging from functional analyses of plant poly(A) signals, allowing me aningful comparison with components of poly(A) signals in other eukary otes. A plant poly(A) signal consists of one or more near-upstream ele ments (NUE), each directing processing at a poly(A) site a short dista nce downstream of it, and an extensive far-upstream element (FUE) that enhances processing efficiency at all sites. By analogy with other sy stems, a model for a plant 3'-end processing complex can be proposed. Plant poly(A) polymerases have been isolated and partially characteris ed. These, together with hints that some processing factors are conser ved in different organisms, opens promising avenues toward initial cha racterisation of the trans-acting factors involved in 3'-end formation of mRNAs in higher plants.