Investigation of the RNA polymerase III (pol III) transcription system of eukaryotes was continued, focusing on pol III transcription factors (TF) and the function of the human La antigen. Pol III produces small RNAs, including the 5S ribosomal RNA and transfer RNAs essential for protein synthesis, as well as certain virus-encoded transcripts. Pol III is a complex enzyme composed of at least seventeen tightly-associated subunits with multiple catalytic activities. Pol III is directed to its target genes by TFs. Human La antigen is a regulatory phosphoprotein that has been shown to serve as a termination factor for pol III, stimulating transcription and directing maturation of the transcripts, the latter of which is a complex operation that includes end-processing, intron removal, base modification, and proper folding. La interacts with these transcripts by recognizing their 3' terminal UUU-OH motifs, which result from pol III termination, found at the ends of all newly synthesized pol III transcripts. Patients suffering from systemic lupus erythematosous (a.k.a., SLE, Lupus), neonatal lupus, and Sjogrens syndrome produce autoimmune antibodies to the La antigen. Some viral-encoded factors, including the adenovirus E1A protein, modulate the activity of specific pol III TFs, and other virus factors modulate the nucleo-cytoplasmic distribution and activity of La. Understanding the mechanisms by which La and other pol III TFs function in RNA production pathways, and how these pathways are controlled during normal cellular proliferation, are major goals of this Section. Experimental systems representing only two organisms are currently available for the study of pol III transcription, the budding yeast, S. cerevisiae, and human. While these share extensive homology in the factors and mechanisms involved in transcription initiation, notable differences exist in the TFs that map to the termination ends of the transcription complex as well as in the minimal termination signals required for pol III termination. Therefore, another goal of this Section is to investigate pol III transcription in the fission yeast, S. pombe, a genetically- tractable organism in which our recent studies suggest a pol III termination signal mechanism more similar to that of humans than is S. cerevisiae. Summary of Major Findings: 1. nonphosphoLa is found in the cytoplasm associated with 5'TOP mRNAs and also in the nucleolus, associated with the protein nucleolin, while phosphoLa is found in the nucleoplasm associated with precursor tRNAs. 2. The RNA polymerase III subunit, Rpc11p, is a molecular determinant of the length of the 3' oligo-U tract that terminates pol III transcripts. 3. Human La antigen promotes the translation of Mdm2 mRNA and leads to progression of chronic myelogenous leukemia.
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