Reduced histone gene copy number disruptsDrosophilaPolycomb function

Abstract

AbstractThe chromatin of animal cells contains two types of histones: canonical histones that are expressed during S phase of the cell cycle to package the newly replicated genome, and variant histones with specialized functions that are expressed throughout the cell cycle and in non-proliferating cells. Determining whether and how canonical and variant histones cooperate to regulate genome function is integral to understanding how chromatin-based processes affect normal and pathological development. Here, we demonstrate that variant histone H3.3 is essential forDrosophiladevelopment only when canonical histone gene copy number is reduced, suggesting that coordination between canonicalH3.2and variantH3.3expression is necessary to provide sufficient H3 protein for normal genome function. To identify genes that depend upon, or are involved in, this coordinate regulation we screened for heterozygous chromosome 3 deficiencies that impair development of flies bearing reducedH3.2andH3.3gene copy number. We identified two regions of chromosome 3 that conferred this phenotype, one of which contains thePolycombgene, which is necessary for establishing domains of facultative chromatin that repress master regulator genes during development. We further found that reduction inPolycombdosage decreases viability of animals with noH3.3gene copies. Moreover, heterozygousPolycombmutations result in de-repression of the Polycomb target geneUbxand cause ectopic sex combs when either canonical or variantH3gene copy number is also reduced. We conclude that Polycomb-mediated facultative heterochromatin function is compromised when canonical and variantH3gene copy number falls below a critical threshold.