The continuity of gene expression programs is in part maintained by the faithful transmission of exiting chromatin structures from the parental cells to the daughter cells. Replication-coupled processes ensure that the epigenetic marks on the old histones (and on DNA in metazoan organisms) are faithfully reproduced after the synthesis of the new strands of DNA and the deposition of new histones. At the same time, changes in chromatin structure are necessary during metazoan development and in the adaptation of S.cerevisiae and many pathogens to the changing or host organism environment. It is not clear if the same machinery is involved in these epigenetic changes between active and silent states of the genes. In reality, we know very little about how epigenetic conversions take place.
Recently we have developed an assay for the quantitative assessment of the frequency of epigenetic conversions at the telomeres of S.cerevisiae. We have documented that the destruction of Chromatin Assembly Factor -1 (CAF-1) or the DNA helicase RRM3 substantially reduce the frequency of such conversions. CAF-I is a histone chaperone, which reassembles nucleosomes from old and new histones immediately after the passage of the replication forks. RRM3 is a helicase that relives replication forks that have paused at sites of tight protein binding or active transcription.
Current models suggest that both Rrm3p and CAF-1 are recruited to replication forks via an interaction with the Proliferating Cell Nuclear Antigen (PCNA, POL30) and that this interaction is regulated by the DBF4-Dependent Kinase, DDK. I will present evidence that the Cyclin Dependent kinase CDK (Cdc28p) rather than DDK is a key regulator of the association of CAF-I with chromatin, while DDK affect its stability and its affinity to PCNA. Two separate posters will present details on these findings.
I will propose the model that CAF-I, albeit being involved on the faithful transmission of epigenetic marks, is also needed for epigenetic conversions at paused replication forks.
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