Fig. 2.

Dual function of TR during Xenopus metamorphosis. TR can repress and activate T3-inducible genes in the absence and presence of T3, respectively. The presence of little or no T3 in premetamorphic tadpoles enables most or all TRs remaining in the unliganded state and TR/RXR heterodimers recruit HDAC-containing corepressors complexes to target genes. This in turn reduces the levels of activation histone marks and increases the levels of repression histone marks, resulting in gene repression. With rising levels of T3 during metamorphosis, TRs become T3-bound and TR/RXR heterodimers recruit coactivator complexes. This then causes chromatin remodeling, including the loss of 2–3 nucleosomes around the TRE and histone modifications, particularly an increase in activation histone marks and a decrease in repression histone marks, and eventually transcriptional activation. N-CoR, nuclear corepressor; HDAC, histone deacetylase; SRC3, steroid receptor coactivator 3; p300, a histone acetyltransferase; PRMT1, protein arginine methyltransferase 1.