| Summary: | 碩士 === 國立陽明大學 === 微生物及免疫學研究所 === 94 === The nucleolus is specialized for the synthesis of ribosomal RNAs (rRNAs) and the assembly of pre-ribosomal subunits. It is generally accepted that the nucleolar formation at sites of the rRNA genes is dependent on the RNA polymerase I (Pol I) transcription. Treacle, a nucleolar protein as a mutation target involved in the Treacher Collins syndrome, is closely related to another nucleolar protein hNopp140, which interacts with Pol I and may play a role in sustaining nucleolar structure. Both treacle and hNopp140 bind to rRNA genes as demonstrated by chromatin immunoprecipitation assays. Cells after overexpressing the wild-type treacle, hNopp140 was redistributed to the newly formed, treacle-containing spherical structure that occupied and enlarged the nucleolus. Treacle without the N-terminal domain had no effects on the formation of the sphere-like structure in the nucleolus. In contrast, a treacle mutant lacking the C-terminal domain was dispersed throughout the nucleoplasm and did not alter the nucleolar localization of hNopp140. Interestingly, ecotopic overexpression of treacle mutants missing the Pol I binding sites still resulted in the chase-out of Pol I from nucleolus. Although the treacle deletion mutant bearing only the N- and C-termini was able to form aggregates in nucleolus and nucleoplasm, it affected the endogenous hNopp140; apparently, hNopp140 had a tendency to reside in the nucleolus in the beginning but gradually to move out with this treacle mutant. When treacle expression levels were knocked down by treacle siRNA, the nucleolar retention of hNopp140, Pol I, upstream binding factor, and rRNA genes was altered or diminished synchronously. These observations imply that all these proteins coexist in a complex and may be functionally linked. Further experiments are needed to define their intrinsical properties in the maintenance of nucleolar structure.
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