Histone H3.3 deposition at E2F-regulated genes is linked to transcription.

The histone variant H3.3 can be incorporated in chromatin independently of DNA synthesis. By imaging using green fluorescent protein-tagged histones, H3.3 deposition has been found to be linked with transcriptional activation. Here, we investigated H3.3 incorporation during G1 progression on cell-cycle-regulated E2F-dependent genes and on some control loci. We transiently transfected resting cells with an expression vector for tagged H3.3 and we analysed its presence by chromatin immunoprecipitation. We found that replication-independent H3.3 deposition occurred on actively transcribed genes, but not on silent loci, thereby confirming its link with transcription. Interestingly, we observed similar levels of H3.3 occupancy on promoters and on the coding regions of the corresponding genes, indicating that H3.3 deposition is not restricted to promoters. Finally, H3.3 occupancy correlated with the presence of transcription-competent RNA polymerase II. Taken together, our results support the hypothesis that H3.3 is incorporated after disruption of nucleosomes mediated by transcription elongation.