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内容記述 |
Relocation of euchromatic genes near a heterochromatin region often results in mosaic gene silencing in eukaryotes. In Saccharotmyces cerevisiae, when a wild-type gene is located near a telomere, it is subjected to telomere position-effect variegation (TPE), which includes transcriptional silencing and which provides heritable silent and expressed states as well as reversible switching between the epigenetic states. The silent state of a telomeric gene is attributable to heterochromatin-like structure, which is composed of several proteins such as Sir-proteins and hypoacetylated histones and which spreads from the telomeric end. While many modifiers of telomeric heterochromatin have been identified, the molecular nature for the switching and heritable propagation of epigenetic states is not well understood. To explain the stochastic nature of phenotypic variegation and stable inheritance of the epigenetic states, it has been proposed that these phenomena are attributed to competition between the assembly of heterochromatin and the establishment of active-chromatin. Because the silent state of a telomeric gene is altered to the expressed state by a trans-activator in G2/M-phase-arrested cells but not in G1- or early S-phase-arrested cells, it has been suggested that progression through the S-phase is required in TPE for switching from the silent to the expressed state. Furthermore, mutant forms of the replication protein PCNA are defective in silencing and interaction with CAF-1, a replication-coupled chromatin assembly factor, suggesting that DNA replication machinery is linked to silencing at heterochromatin.<br /> One of replicative polymerases, DNA polymerase ε (Pol ε), is composed of the catalytic-subunit Pol2, Dpb2, Dpb3 and Dpb4 in S. cerevisiae, and the feature of the subunit composition of Pol ε is evolutionally conserved in eukaryotes. Although Pol2 and Dpb2 are essential for DNA replication, Dpb3 and Dpb4, which contain the histone-fold motif related to chromatin metabolisms, are dispensable, and their function is not clear. To gain insight into possible roles of Pol ε in chromatin configuration, I examined TPE in mutant cells defective in Pol ε.<br /> In the assay of silencing with telomeric URA3, dpb3Δ, dpb4Δ and pol2-11 (C-terminal mutant of POL2) cells displayed a partial defect of silencing, and this defect was most evident in dpb3Δ cells. The silencing defect of pol2-11 cells was completely suppressed by simultaneous introduction of high copy DPB3 and DPB4. Moreover, the silencing level in the dpb3Δ dpb4Δ double mutant was similar to that in dpb4Δ, indicating that the dpb4Δ mutation is epistatic to the dpb3Δ mutation. In parallel, I also observed the expression of telomeric ADE2, which gave rise to red and white sector colonies in the wild-type strain because of mosaic gene silencing by TPE. I found that dpb4Δ mutant cells form non-sectoring light pink colonies and dpb3Δ mutant cells form white colonies. These results suggest that the mutations in Pol ε increase the switching frequency between alternative epigenetic states in TPE.<br /> To monitor the switching between a silent and an expressed state in each cell division, I developed a single-cell telomeric silencing assay, with which I could distinguish between a silent state (off) and an expressed state (on) of telomeric α2 gene in a single cell on an α-factor-containing medium. With this assay, I measured switching rates from "on" to "off' and from "off' to "on", and found that both switching rates increased in dpb4Δ cells, whereas in dpb3Δ cells the switching rate horn "off' to "on" specifically increased. These results suggest that Dpb4 plays a role in the stable inheritance of both silent and expressed states, while Dpb3 is involved only in the stable inheritance of a silent state.<br /> The epistasis of dpb4Δ to dpb3Δ, together with different switching patterns in these mutants, suggests that Dpb4 is shared by Pol ε and an unknown complex that plays a counteracting role in TPE. I thus purified protein complexes containing Dpb4 using anti-Flag antibody and the 5Flag-epitope tagged Dpb4 protein, and found that two distinct protein complexes, Pol ε and yCHRAC, share Dpb4. yCHRAC is a putative homologue of the chromatin accessibility complex CHRAC in higher eukaryotes, and composed of a WAC-motif protein Itc1, an ISWI-chromatin remodeling factor homologue Isw2, a novel histone-fold protein Dpb31, and Dpb4. Since Pol ε and CHRAC in human cells also share a histone-fold protein that is a counterpart of Dpb4, it is suggested that the relationship between Pol ε and the CHRAC-like complex is evolutionally conserved.<br /> I next addressed whether yCHRAC counteracts Pol ε for TPE. In the assays with telomeric URA3 and ADE2, the itc1Δ and dpb31Δ mutations enhanced telomere silencing and restored it in dpb3Δ cells to the level of dpb4Δ cells, whereas they did not affect it in dpb4Δ cells. Moreover, in contrast to the dpb3Δ mutation, the dpb31Δ mutation increased the switching rate from "on" to "off', but did not affect that from "off' to "on". Therefore, these results suggest that yCHRAC regulates the stable inheritance of the expressed state of TPE independent of Pol ε, while yCHRAC and Pol ε counteract each other for TPE.<br /> In conclusion, position effect variegation at yeast telomeres is caused not only by simple competition between the assembly of heterochromatin and the establishment of active-chromatin but also by specific factors such as Pol ε and yCHRAC, which serve for the stable inheritance of the epigenetic states. |
要旨・審査要旨 / Abstract, Screening Result (385.5 kB)
