Nucleosome-specific, time-dependent changes in histone modifications during activation of the early growth response 1 (Egr1) gene.

Histone post-translational modifications and nucleosome remodeling are coordinate events involved in eukaryotic transcriptional regulation. There are relatively few data on the time course with which these events occur in individual nucleosomes. As a contribution to fill this gap, we first describe the nature and time course of structural changes in the nucleosomes -2, -1, and +1 of the murine Egr1 gene upon induction. To initiate the transient activation of the gene, we used the stimulation of MLP29 cells with phorbol esters and the in vivo activation after partial hepatectomy. In both models, nucleosomes -1 and +1 are partially evicted, whereas nucleosomes +1 and -2 slide downstream during transcription. The sliding of the latter nucleosome allows the EGR1 protein to bind its site, resulting in the repression of the gene. To decide whether EGR1 is involved in the sliding of nucleosome -2, Egr1 was knocked down. In the absence of detectable EGR1, the nucleosome still slides and remains downstream longer than in control cells, suggesting that the product of the gene may be rather involved in the returning of the nucleosome to the basal position. Moreover, the presence of eight epigenetic histone marks has been determined at a mononucleosomal level in that chromatin region. H3S10phK14ac, H3K4me3, H3K9me3, and H3K27me3 are characteristic of nucleosome +1, and H3K9ac and H4K16ac are mainly found in nucleosome -1, and H3K27ac predominates in nucleosomes -2 and -1. The temporal changes in these marks suggest distinct functions for some of them, although changes in H3K4me3 may result from histone turnover.