The number of molecules in a cell is finite, but whether cellular processes compete for these limited molecular resources is not well understood. Now, Patrick Murphy and colleagues investigate this concept in the context of H2A.Z histone distribution in zebrafish chromatin. H2A.Z is associated with active transcription; however, H2A.Z is also found in non-genic repetitive elements, such as transposons, which are typically repressed but can become de-repressed in response to stress. To understand whether repetitive element de-repression relies on H2A.Z, and reveal the consequences H2A.Z recruitment to these sites, the researchers use stress-mimicking chemical treatments combined with epigenetic profiling (such as ATAC-seq and CUT&Tag) to track H2A.Z distribution. They show that, in response to chemical treatment, H2A.Z enrichment decreases at gene promoters, reducing gene expression, but increases in repetitive element-containing regions, which is accompanied by their de-repression. The researchers then transgenically over-express H2A.Z to increase its abundance and show that it becomes distributed over both genic and intergenic sites. Together, these data show that DNA elements compete for the limited abundance of H2A.Z and that the de-repression of repetitive elements acts as a sink, titrating the histone away from genes and repressing their expression.
