Genes positioned close to telomeres are either fully active or fully silenced and infrequently convert between these two states, a phenomenon referred to as Telomere Position Effect. The mechanisms of epigenetic conversions at the telomeres, and in general, are not well understood. Recently we have shown that the destruction of Chromatin Assembly Factor – I (CAF-I) in S.cerevisiae reduces the frequency of such telomeric conversions. CAF-I is a histone chaperone which rebuilds nucleosomes from old and new histones immediately after the passage of the replication forks. It is believed that its nucleosome assembly activity is central to the transmission of epigenetic marks to the chromatin of daughter cells and that it acts as a key factor in the preservation of epigenetic states.
In this poster we show that the deletion of RRM3, a DNA helicase that releases paused replication forks, has a similar effect on the frequency of epigenetic conversions. We present our analyses on the occupancy of CAF-1 and Rrm3p at replication forks that are stably paused in vivo by the association of the Fob1p protein. We correlate these Chromatin Immuno-Precipitation (ChIP) analyses with analysis of the interactions of Rrm3p and CAF-1 with the fork clamp Pol30p (PCNA, Proliferating Cell Nuclear Antigen).
We propose that CAF-I, albeit being involved on the faithful transmission of epigenetic marks, is needed for epigenetic conversions when replication forks stall.
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