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In Saccharomyces cerevisiae, the silent mating loci are repressed by their assembly into heterochromatin. The formation of this heterochromatin requires a cell cycle event that occurs between early S phase and G2/M phase, which has been widely assumed to be DNA replication. To determine whether DNA replication through a silent mating-type locus, HMRa , is required for silencing to be established, we monitored heterochromatin formation atHMRa on a chromosome and on a nonreplicating extrachromosomal cassette as cells passed through S phase. Cells that passed through S phase established silencing at both the chromosomalHMRa locus and the extrachromosomalHMRa locus with equal efficiency. Thus, in contrast to the prevailing view, the establishment of silencing occurred in the absence of passage of the DNA replication fork through or near theHMR locus, but retained a cell cycle dependence.
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