Histone acetylation plays an important role in the assembly and regulation of chromatin structure. The transfer of acetyl groups onto histone lysine residues is catalyzed by Histone cetyltransferases (HATs) of which there are two classes, type-A and type- B. The type-A HATs catalyze the acetylation histone in a chromatin context and are involved in transcriptional regulation. The type-B HATs are specific for the acetylation of free histones and are thought to function in chromatin assembly. The only Type-B HAT identified to date is Hat1p.
It has been shown that deletion of hat1 in combination with specific lysine to arginine mutations in the histone H3 tail, results in a defect in telomeric silencing. However, the exact mechanism by which Hat1p participates in telomeric silencing remains unknown.
In the current study, we set out to further our understanding of the mechanism by which Hat1p is involved in telomeric silencing in yeast. First we examined the role of Hat1p in maintenance of in telomere tract length. In this study we demonstrate that the telomeric silencing defect observed in the hat1/histone H3 K14,23R strain is not the result of telomere shortening. We also provide evidence that Ha1p is not stably associated with telomeres and therefore it is unlikely to mediate telomeric silencing by directly modifying histone H4 that is present near telomeres.
Next, we investigated whether Hat1p functions in a pathway with known histone chaperones Hir2 and Asf1. Combined mutations in histone H3, hat1, and hir2 or asf1 resulted in synergistic affects on telomeric silencing. Therefore, Hat1p functions in a pathway that appears to be independent of and functionally redundant with a pathway(s) involving Hir2 and Asf1. In addition, the ability of Hat1p to localize to the nucleus is found to be vital to its role in telomeric silencing.
Next, we examined the functional relationship between an essential subunit of the FACT complex, histone tail modification sites, and Hat1p. We provide evidence that a viable point mutant in a FACT subunit, pob3Q308K, tolerates hat1 deletion or individual lysine to arginine substitution at histone H4 positions 5 or 12, but displays a growth phenotype when both K5 and K12 are simultaneously mutated to arginine. In addition, an additive growth defect was observed when pob3Q308K, hat1 deletion, and histone H3K5,12R were combined. This serves as the first evidence suggesting that Hat1p may play a role in the cell other than acetylating H4 K12.
Lastly, the pob3Q308K strain displayed sensitivity to lysine to arginine substitution at positions 14, 23, and 27 of the histone H3 tail. Given the sensitivity of the pob3 point mutant to specific histone H4 mutations that abolish the evolutionary conserved pattern of acetylation involved in chromatin assembly, the sensitivity of this strain to specific mutants in the H3 tail suggests that these modification sites may also play a role in chromatin assembly.
Taken together the data provided in this study further our understanding of the structural and functional characteristic of Hat1p.
|Commitee:||Charles, Bell, Guttridge, Denis, Ma, Jiyan|
|School:||The Ohio State University|
|School Location:||United States -- Ohio|
|Source:||DAI-B 78/11(E), Dissertation Abstracts International|
|Subjects:||Biology, Biochemistry, Biomedical engineering|
|Keywords:||Chromatin, Histone acetyltransferase, Yeast|
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