Uridylate-Rich Small Nuclear RNAs (UsnRNAs), Their Genes and Pseudogenes, and UsnRNPs in Plants: Structure and Function. A Comparative Approach

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Abstract
Uridylate-rich small nuclear RNAs (UsnRNAs) including the major, or abundant (U1-U6), and the minor, or low abundant (U7-U14), UsnRNAs are a distinct class of evolutionarily highly conserved, metabolically stable molecules that are indispensable functional components (in the form of RNP particles, the UsnRNPs) or nuclear RNP complexes in which splicing of pre-mRNA (U1, U2, U4-U6 RNAs) or processing of pre-rRNA (U3, U8, U13, U14 RNAs) takes place. Their genes are transcribed either by RNA 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 (nucleolus) to exert their function there. Inclusion of plant systems into studies of the structure and function of UsnRNAs, of their genes, and of UsnRNPs (1) substantially contributed to the identification of phylogenetically conserved primary and secondary structural elements in the major UsnRNAs, thus permitting the establishment of the “functional anatomy” of the individual UsnRNA species and pointing to the overall similarity of the function of UsnRNAs and UsnRNPs in pre-mRNA splicing and pre-rRNA processing in eukaryotes; (2) revealed some “plant-specific” aspects of UsnRNA structure (Sm-binding site, cap, and secondary 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 III), 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., characterization of low abundance UsnRNAs, structure of plant UsnRNPs, tarns-acting factors involved in the transcription of plant U-genes, preparation of in vitro splicing systems).

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