During heat shock, cells respond to temperature stress by opening their chromatin, allowing the transcription of genes that enable them to cope with the sudden change in the environment. How chromatin becomes decondensed to permit active transcription during this process is not clear. Recently, a model based on experiments in Drosophila has been proposed, whereby histone H2Av is deposited and subsequently phosphorylated by JIL-1 kinase, followed by recruitment of poly(ADP-ribose) polymerase 1 (PARP-1). PolyADP-ribosylation of chromatin then takes place, which loosens its structure. This permits phosphorylation of serine 10 in the tail of histone H3 (H3S10p), again by JIL-1 kinase, which is required for the function of the transcriptional machinery. On p. 3232, Kristen Johansen and colleagues test this model using null mutants and find that H2Av phosphorylation and chromatin opening can occur in the absence of JIL-1 kinase, and that H3S10p still occurs in a PARP-1 knockdown mutant. In light of these findings, the proposed model breaks down. Instead, the authors find that PARP-1 can be recruited by H3S10p independently of H2Av, providing insight into an alternative mechanism for opening up of chromatin structure to permit active transcription in Drosophila.
