Two HIRA dependent pathways mediate H3.3 de novo deposition and recycling during transcription
Nucleosomes represent a challenge in regard to transcription. Histone eviction enables RNA polymerase II (RNAPII) progression through DNA, but compromises chromatin integrity. Here, we used the SNAP tag system to distinguish new and old histones and monitor chromatin reassembly coupled to transcription in human cells. We uncovered a transcription dependent loss of old histone variants H3.1 and H3.3. At transcriptionally active domains, H3.3 enrichment reflected both old H3.3 retention and new deposition. Mechanistically, we found that the histone regulator A (HIRA) chaperone is critical to processing both new and old H3.3 via different pathways. De novo H3.3 deposition is totally dependent on HIRA trimerization as well as on its partner ubinuclein 1 (UBN1) while antisilencing function 1 (ASF1) interaction with HIRA can be bypassed. By contrast, recycling of H3.3 requires HIRA but proceeds independently of UBN1 or HIRA trimerization and shows absolute dependency on ASF1 HIRA interaction. We propose a model whereby HIRA coordinates these distinct pathways during transcription to fine tune chromatin states. A SNAP tag approach to monitoring histone dynamics at transcriptionally active sites in human cells identifies two distinct HIRA dependent pathways that direct H3.3 deposition or retention.
Nature structural molecular biology