| Research Abstract |
Transcirpional activation or repression in eukaryotes is believed to be accompanied by an alteration of chromatin structure. In recent years, factors which may affect nucleosome or higher-order chromatin structure begun to be unveiled in several organisms including yeast Saccharomyces cerevisiae. In this study, we focused on one of such factors, SIN4 of S.cerevisiae. Our previous study has defined two classes of promoters, those which are activated by the sin4-mutation and those which are not activated by the sin4-mutation. We analyzed the mechanisms of this differential response of the two classes of promoters to thesin4 mutation. Results of our analysis revealed that the sin4 mutation activates core promoter-dependent basal transcription but not activator-mediated transcription. From this and other observations, we suggest that activation of the basal transcription occurs through a mechanism different from activator-mediated transcriptional enhancement. In sebsequent study, we identi
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fied TUP1 and SSN6 as negative regulators of transcription of IME1, a transcriptional activator of meiosis in S.cerevisiae. It has been well established that the TUP1 and SSN6 is recruited by a gene-specific DNA-bining protein to act as a general repressor of various genes by facilitating the rigid organization of nucleosome structure. Rind is known to be a DNA-binding protein which binds to upstream of IME1 to repress the IME] transcription. However, deletion of the Rme1-binding site from IME1 promoter did not result in activation of the expression of IME1, indicating that Rme1 does not function as a DNA-binding protein to recruit the Tup1-Ssn6 repressor complex. We also examined the effect of disruption of SIR2, SIR3 and HHO1 in addition to TUP1 and Sin4, all of which are thought to repress transcription through altering chromatin structure, on transcription of all genes on chromosome VI of S.cerevisiae. Results showed that enhancement of transcription of the HSP12 and HXK1 was seen in the sir2 and sir3 disruptant. However, transcriptional enhancement of those genes ocurred without that of their adjacent genes, suggesting that chromatin-mediated repression is gene-specific event. Increased accessibility of Micrococcal nuclease to the promoter regions of HSP12 and HXK1 in the sir3 disruptant suggested that heterochromatin-like structure exists in the internal regions of the chromosome VI in addition to well-known HM loci and telomeric regions. Less
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