S phase inheritance of transcriptional silencing: The role of Sir1 and Asf1

To preserve the transcriptional program of dividing cells, silent and active chromatin domains must not only be continuously maintained but also faithfully inherited. Cell cycle progression, however, poses significant potential challenges to the inheritance of silent chromatin states. The chromatin structure undergoes major structural alterations in S and M phase and these alterations likely affect the higher order heterochromatin chromatin structure. In a cell cycle, however, changes in defined transcriptional programs rarely occur. Thus, if DNA replication and Mitosis disrupt silent chromatin, then there must be factors that restore silencing immediately following its disruption. In the budding yeast, Saccharomyces cerevisiae, I studied how transcriptionally silent states are inherited. I identified for the first time two proteins required for the inheritance of silencing. I demonstrate that these two proteins, Sir1, a silencer associated protein, and Asf1, a nucleosome deposition factor, are S phase specific inheritance factors. Using sir1td and asf1td conditional alleles, I also demonstrate that the S phase loss of silencing correlates with DNA replication of the silent locus, suggesting that DNA replication is the S phase event that disrupts transcriptional silencing. Finally, I show that the core silencing protein, Sir3, remains associated with HMLalpha for at least three generations after silencing is lost. This indicates that Sir3 may template the inheritance of silent chromatin following S phase