The small nuclear RNAs known as U1, U2, U3, U4, U5, and U6 comprise a highly abundant class of RNA molecules present in the nuclei of higher organisms. The roles of these evolutionarily conserved molecules have not yet been clearly defined, but evidence is accumulating that they are involved in processing (or transporting) nuclear precursors of cytoplasmic messenger and ribosomal RNAs. Multiple copies of the genes coding for the snRNAs are known to exist in animal genomes, and apparently unique mechanisms are involved in the expression of the snRNA genes. Current evidence suggests that the mature snRNAs are most likely generated from larger precursors by means of a series of processing steps that are at the present time still unknown and uncharacterized. Alternately, it is possible that unique nucleotide sequences in the DNA and protein factors, specific for the initiation of transcription from snRNA genes, may be required. In order to determine the mechanisms and exact pathways involved in the synthesis of snRNAs, the expression of chicken U1 RNA genes will be concentrated upon and studied in detail. First, the site(s) of transcriptional initiation will be precisely mapped both in vivo and in vitro using a variety of methods. Next, nucleotide sequences required for the initiation of U1 RNA gene transcription will be determined. If unique protein transcription factors appear to be involved, these proteins will be characterized in terms of function. Similarly, nucleotide sequences necessary for the accurate processing of U1 RNA primary transcripts will be identified. Moreover, the enzymatic activities responsible for processing the primary transcript of mature U1 RNA will be purified, and the mechanisms of action of these enzymes will be investigated.
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