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A NEW FORM OF GENE GATING
Sun Yat-sen University, Guangzhou, China
Thesis Advisor: Marc R. Gartenberg, PhD
Graduate Program in Cellular and Molecular Pharmacology
4th Floor Conference Room, Dept of Pharmacology
Thursday, April 11, 2013
Budding yeast tRNA genes are located throughout the genome. Nevertheless, large families of the genes cluster together at the nucleolus in a condensin and microtubule dependent manner. While investigating the nuclear position of one well-studied tRNA gene named tT(AGU)C, it was noticed that the gene rarely colocalized with the nucleolus. Using fluorescence microscopy and chromatin immunoprecipitation, I found instead that the gene associated with nuclear pore complexes, and that this localization occurred primarily in M Phase of the cell cycle. Two other representative tRNA genes, tS(CGA)C and tT(UGU)G1 displayed similar characteristics, indicating that NPC contact is a property common to other and perhaps all tRNA genes. Association of tRNA genes with NPCs required nucleoporins, the DNA sequences of the tRNA gene, microtubules and cohesin, the protein complex responsible for holding sister chromatids together. Small DNA circles bearing tT(AGU)C also associated with NPCs, indicating that the gene need not be physically-linked to the rest of the chromosome for contact to occur. Importantly, NPC-tRNA gene contact correlated with increased binding of RNA polymerase III at tRNA genes and elevated nascent tRNA levels. Moreover, enhancing tRNA gene transcription by deleting the negative regulator MAF1 increased NPC-tRNA gene contact outside of M phase. Collectively, these data show that activation of tRNA genes leads to their association with NPCs, much like the behavior of many activated RNA polymerase II transcribed genes. I propose that active tRNA genes associate with NPCs to facilitate export of tRNAs to the cytoplasm, as predicted in the gene-gating hypothesis.