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G.–H. Peng, S. Chen; Correlated Crx Target Binding, Histone Acetylation and Photoreceptor Gene Transcription in the Developing Mouse Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3130.
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© ARVO (1962-2015); The Authors (2016-present)
Active transcription of the photoreceptor genes requires both "de–repression" and activation mechanisms. "De–repression" often involves activator–dependent chromatin modifications, such as the acetylation of histones, which "opens" chromatin structure, allowing activators and the transcriptional machinery to access their target genes. We reported previously (2004 ARVO, Abstract 2253) that the homeodomain transcription factor Crx is required for the acetylation of histone H3 on photoreceptor gene chromatin by recruiting co–activator complexes containing histone acetyl–transferases (HATs). The purpose of this study was to examine the temporal patterns of Crx binding and histone H3 acetylation on selected photoreceptor gene chromatin during development, and to compare these patterns with the timing of transcriptional initiation of these target genes.
Retinas of C57/BL6 mice at various developmental ages, from E16.5 to P7, were examined using chromatin immunoprecipitation (ChIP) with antibodies against photoreceptor transcription factors (Crx, Nrl, and Nr2e3), acetylated histone H3 (AcH3) and an active form of RNA polymerase II (Pol II). PCR with selected gene–specific primers was used to analyze the immunoprecipitated chromatin DNA. RT–PCR was also performed to examine target gene transcription.
Crx associates with the promoters of cone opsins and cone arrestin (Arr3) starting at E18.5. This is significantly earlier than when Crx associates with the promoters of the rod genes, rhodopsin, Pde6a and Pde6b (starting at P3). However, Crx binds to the enhancer regions of cone opsins at around P3, similar to when it binds to the rhodopsin promoter/enhancer. Acetylation of histones on the cone and rod gene promoters and enhancers does not occur until after birth. This increase in AcH3 coincides with the binding of Nrl, Nr2e3, and Pol II. AcH3 levels reach a peak around P5, followed by the peak of transcriptional initiation of cone and rod genes.
Based on these findings and our previous data, the binding of Crx to the regulatory regions of its target genes is likely to trigger histone acetylation and de–repression. This may allow the binding of Nrl, Nr2e3 and RNA polymerase II to the target photoreceptor genes, leading to their transcriptional activation.
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