Abstract
In preparation for transcription, the chromatin remodeler SWR installs homotypic ZZ nucleosomes at promoters by replacing the two nucleosomal H2A with H2A.Z in a stepwise manner. Nucleosome-free regions (NFRs) help recruit SWR to promoters; this is thought to position SWR asymmetrically on one side of the +1 nucleosome. How SWR accesses the opposite side of +1 to generate a ZZ nucleosome remains unclear. Using biochemical assays that monitor the sub-nucleosomal position of nascent H2A.Z, we find that NFR-recruited SWR switches sides to insert H2A.Z into asymmetrically positioned nucleosomes; however, at decreasing temperatures, H2A.Z insertion becomes progressively biased for one side. We find that a 16-bp element containing G/C runs (>3 consecutive G or C nucleotides) is sufficient to promote H2A.Z insertion. Because H2A.Z-rich +1 nucleosomes in yeast have more G/C runs, we propose that nucleosome editing is a thermosensitive process that can be hard coded by the genome.
Keywords:
H2A.Z; NFR; SWR1; chromatin remodeling; histone exchange; nucleosome; temperature; yeast.