URIDYLATE-RICH SMALL NUCLEAR RNAS (USNRNAS), THEIR GENES AND PSEUDOGENES, AND USNRNPS IN PLANTS - STRUCTURE AND FUNCTION - A COMPARATIVE APPROACH

Uridylate-rich small nuclear RNAs (UsnRNAs) including the major, or ab undant (U1-U6), and the minor, or low abundant (U7-U14), UsnRNAs are a distinct class of evolutionarily highly conserved, metabolically stab le molecules that are indispensable functional components (in the form of RNP particles, the UsnRNPs) or nuclear RNP complexes in which spli cing of pre-mRNA (U1, U2, U4-U6 RNAs) or processing of pre-rRNA (U3, U 8, U13, U14 RNAs) takes place. Their genes are transcribed either by R NA polymerase II (U1-U5 RNAs in metazoa) or by RNA polymerase III (U6 RNA), requiring very specific transcription signals. Most UsnRNPs are assembled in the cytoplasm and then are transported to the nucleus (nu cleolus) to exert their function there. Inclusion of plant systems int o studies of the structure and function of UsnRNAs, of their genes, an d of UsnRNPs (1) substantially contributed to the identification of ph ylogenetically conserved primary and secondary structural elements in the major UsnRNAs, thus permitting the establishment of the ''function al anatomy'' of the individual UsnRNA species and pointing to the over all similarity of the function of UsnRNAs and UsnRNPs in pre-mRNA spli cing and pre-rRNA processing in eukaryotes; (2) revealed some ''plant- specific'' aspects of UsnRNA structure (Sm-binding site, cap, and seco ndary structure of U3 RNA, exceptional abundance of sequence variants) , expression of UsnRNA genes (plant-specific transcription signals and transcription of plant U3 genes by RNA polymerase 111), and pre-mRNA splicing (functionally significant features of plant introns); and (3) highlighted some areas of the field in which the use of plant systems compared to mammalian and yeast systems is lagging behind (e.g., char acterization of low abundance UsnRNAs, structure of plant UsnRNPs, tra ns-acting factors involved in the transcription of plant U-genes, prep aration of in vitro splicing systems).