Small nuclear RNAs in messenger RNA and ribosomal RNA processing.

Ribonucleoproteins (RNPs) play essential roles in many aspects of gene expression. Two families of nuclear RNPs are involved in the processing of primary transcripts made by RNA polymerases I and II (pol I and II), two of the three polymerases present in the nuclei of eukaryotic cells. Ribosomal RNA precursor transcription by pol I, subsequent processing of the precursor, and the initial steps of ribosome assembly all take place in the nucleolus. A group of nucleolar RNPs containing small RNAs (small nucleolar RNAs or snoRNAs) are involved in the posttranscriptional nucleolar events of ribosome biosynthesis. Six members of a related family of small nuclear RNAs (snRNAs) are required for the processing of mRNA precursors in the nucleoplasm. Five of these snRNAs (U1, U2, U4-6) participate in the removal of intervening sequences while the sixth (U7) plays an essential role in the 3' processing of a subset of mRNA precursors, the histone pre-mRNAs. This is a review of structural and functional aspects of the U1-U7 snRNAs and of snoRNAs.