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Formation of facultative heterochromatin is restricted to the IRER, without reaching the reaper promoter and proximal enhancer regions. (A) Schematic diagram of the intergenic region between reaper and sickle. The IRER is required to mediate irradiation-induced expression of the proapoptotic genes reaper, hid, and sickle (54). Accessibility of the IRER is controlled by a PcG protein-dependent mechanism, which forms a nonpermissive structure in irradiation-resistant cells in post-stage-12 embryos. The decrease of DNA accessibility, accompanied by enrichment of repressive histone marks and binding of PcG proteins, was specifically limited to the IRER without affecting the reaper promoter and proximal enhancer region (54). (B) ChIP assays performed with wild-type adult fly tissues. Pericentromeric heterochromatin locus H23 (H23) and the Ubx promoter region (Ubx) were used as positive controls for repressive histone marks H3K9me3 and H3K27me3, respectively. The coding region of the housekeeping gene Act5C was used as a background control. Enrichment of the repressive histone marks in the IRER was normalized against the respective positive controls and is presented as means ± standard deviations (SD). The high-level enrichment of both H3K27me3 and H3K9me3 in the central part of the IRER dropped significantly at the left boundary of the IRER, about −2 kb to −5 kb relative to the reaper TSS. (C) Distribution of histone marks H3K4me3 and H3K27me3 and binding of Pol II in Drosophila S2 cells revealed by ChIP-Seq. The relative locations of reaper and sickle are indicated by arrows. The dotted vertical line denotes the region that might possess putative chromatin barrier activity.

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