Transcription in the absence of histone H3.3

Di- and trimethylation of histone H3 lysine 4 (H3K4me2 and H3K4me3) are hallmarks of chromatin at active genes. The major fraction of K4-methylated histone H3 is the variant H3 (termed H3.3 in Drosophila), which replaces canonical H3 (H3.2) in transcribed genes. Here, we genetically address the in vivo significance of such K4 methylation by replacing wild-type H3.3 with a mutant form (H3.3K4A) that cannot be methylated. We monitored the transcription that occurs in response to multiple well-described signaling pathways. Surprisingly, the transcriptional outputs of these pathways remain intact in H3.3K4A mutant cells. Even the complete absence of both H3.3 genes does not noticeably affect viability or function of cells: double mutant animals are viable but sterile. Fertility can be rescued by K4-containing versions of H3.3, but not with mutant H3.3 (H3.3K4A) or with canonical H3.2. Together, these data suggest that in Drosophila, presence of H3.3K4me in the chromatin of active genes is dispensable for successful transcription in most cells and only plays an important role in reproductive tissues.