Increasing evidence indicates that transcriptionally silent chromatin structure is dynamic and may change its conformation in response to external or internal stimuli. We show that growth temperature affects all three forms of transcriptional silencing in Saccharomyces cerevisiae. In general, increasing the temperature within the range of 23-37 degrees C strengthens HM and telomeric silencing but reduces rDNA silencing. High temperature (37 degrees C) can suppress the silencing defects of histone H4 mutants. We demonstrate that DNA at the silent HML locus becomes more and more negatively supercoiled as temperature increases in a Sir-dependent manner, which is indicative of enhanced silent chromatin. This enhancement of silent chromatin is not dependent on silencers and therefore does not require de novo assembly of silent chromatin. We also present evidence suggesting that MAP kinase-mediated Sir3p hyperphosphorylation, which plays a role in regulating silencing in response to certain stress conditions, is not involved in high temperature-induced strengthening of silencing. In addition, Pnc1p, a positive regulator of Sir2p activity, plays no role in thermal regulation of silencing. Therefore, growth temperature regulates transcriptional silencing by a novel mechanism.

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