Chromatin regulation by histone chaperone Asf1

Abstract

Asf1 is a conserved H3/H4 histone chaperone with multiple functions in chromatin modulation. Using budding yeast as a model, we identify new pathways of Asf1 function, and expand current knowledge regarding the known roles of Asf1. First, we uncover novel genetic interactions between ASF1 and genes encoding other chromatin regulators. We describe an interaction between ASF1 and SET2, which encodes a histone lysine methyltransferase, and show that Asf1 exists in a pathway to promote Set2-catalyzed H3K36 trimethylation. Second, we present evidence that Asf1 promotes transcriptional derepression of two DNA damage response (DDR) genes (RNR3 and HUG1), which is a hallmark event in the cellular response to replication stress or DNA damage. While Asf1 association with chromatin increases globally during replication stress, we find that direct binding of Asf1 to DDR gene promoters is not needed for their transcriptional derepression. Rather, the contribution of Asf1 is dependent on its ability to stimulate acetylation of H3K56 by the Rtt109 lysine acetyltransferase. This modification occurs in the globular domain of H3, is present on all newly synthesized histones and is lost after H3 incorporation into chromatin. Importantly, DDR gene promoters are occupied by H3K56-acetylated nucleosomes under repressing conditions, and the steady state level of H3K56 promoter acetylation does not change upon derepression. We propose that replication-coupled deposition of K56-acetylated H3 poises newly synthesized DDR genes for derepression. In this model, the known association of transcriptional repressors with DDR gene promoters would ensure that transcription remains minimal under normal conditions. Thirdly, we identify new functions for known Asf1 motifs and characterize key determinants for the constitutive and inducible association of Asf1 with chromatin. Finally, we identify new genetic, physical, and functional links between ASF1 and SNF2, the catalytic subunit of the SWI/SNF chromatin remodeller. Altogether, the research described in this report is consistent with the idea that Asf1 promotes genome stability through its ability to function in diverse pathways of chromatin regulation.