Fig. 7.

A and B, Of the two histone marks associated with gene silencing, H3K9me3 and H3K27me3, only K3K27me3 was reduced upon gene activation during natural metamorphosis. The ChIP assay was performed as in Fig. 5 on intestines isolated from tadpoles at the indicated stages during metamorphosis with anti-H3K27me3 (A) or anti-H3K9me3 antibody (B). ChIP signals were normalized with the ChIP signals of histone H3 in Fig. 5 for the corresponding promoter/exon regions. Note that surprisingly but in agreement with T₃-induced metamorphosis, the amount of H3K9me3 was increased in the downstream transcribed region of TRβ gene (exon 5) upon gene activation during natural metamorphosis (stages 58–62). Error bars indicate sem. C, A model for nucleosome removal and histone modifications during gene activation by liganded TR. In the absence of T₃, TR recruits corepressor complexes containing histone deacetylases, which results in a repressed chromatin state (dashed arrow) with many repression histone marks such as H3K27me3 (triangles). The presence of T₃ induces a conformational change in TR, leading to the recruitment of coactivator complexes containing chromatin remodeling and histone modification enzymes. This results in nucleosomal removal at the promoter region, a reduction in repression histone marks, an increase in activation histone marks such as H3K4me3, H3K7me3, R17me2a, AcH3, and AcH4 (circles), and gene activation (thick arrow).