Nrd1, Nab3, and the nuclear exosome: Transcription termination and RNA degradation.

摘要

The yeast RNA-binding proteins Nrd1 and Nab3 are required for termination of non-polyadenylated transcripts from RNA polymerase II transcribed snRNA and snoRNA genes. Nrd1 and Nab3 have also been shown to regulate certain mRNAs including members of the iron regulon and the NRD1 gene itself. Part I of this dissertation describes the role of Nrd1 and Nab3 in regulating the NRD1 gene. Cis-elements located throughout the NRD1 mRNA are necessary for directing premature transcription termination of this gene. Removal of these elements leads to increased RNA polymerase II occupancy in the 3' end of the gene. A nuclear exosome-activating complex is also involved in directing premature termination, suggesting that degradation of the prematurely terminated transcripts is coupled to transcription termination.; Part II of this dissertation describes the role of Nrd1 and Nab3 in directing transcription termination of "cryptic unstable transcripts", intergenic and antisense transcripts that are synthesized and then rapidly degraded by the nuclear exosome. In nrd1 and nab3 mutant strains long 3'-extended read-through RNAs accumulate from cryptic promoters located throughout the genome. Chromatin immunoprecipitation experiments reveal that Nrd1 and Nab3 co-localize to regions of the genome containing these transcripts. Cloning a termination element from a cryptic unstable transcript has revealed a minimal element sufficient for Nab3-directed termination. Taken together, this dissertation suggests a novel role for Nrd1 and Nab3: transcription termination and subsequent RNA degradation by the nuclear exosome.