Drosophila melanogasterSet8 and L(3)mbt function in gene expression independently of histone H4 lysine 20 methylation

Abstract

Mono-methylation of Lysine 20 of histone H4 (H4K20me1) is catalyzed by Set8 and thought to play important roles in many aspects of genome function that are mediated by H4K20me-binding proteins. We interrogated this model in a developing animal by comparing in parallel the transcriptomes ofSet8null,H4K20R/A, andl(3)mbtmutantDrosophila melanogaster. We found that the gene expression profiles ofH4K20AandH4K20Rlarvae are markedly different thanSet8nulllarvae despite similar reductions in H4K20me1.Set8nullmutant cells have a severely disrupted transcriptome and fail to proliferatein vivo, but these phenotypes are not recapitulated by mutation ofH4K20indicating that the developmental defects ofSet8nullanimals are largely due to H4K20me1-independent effects on gene expression. Further, the H4K20me1 binding protein L(3)mbt is recruited to the transcription start sites of most genes independently of H4K20me even though genes bound by L(3)mbt have high levels of H4K20me1. Moreover, both Set8 and L(3)mbt bind to purified H4K20R nucleosomesin vitro. We conclude that gene expression changes inSet8nullandH4K20mutants cannot be explained by loss of H4K20me1 or L(3)mbt binding to chromatin, and therefore that H4K20me1 does not play a large role in gene expression.