Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 205

DNA

Nucleosome

Heterochromatin

Coactivator complexes
Loss of H1
Histone modifications
e.g., acetylation, phosphorylation, methylation

Histone methylation
Histone deacetylation
Corepressor complexes

Euchromatin

Histone modifications

Figure 4.3 Schematic representation of transition euchromatin-heterochromatin.
Source : From Adcock et al. (2006) .

Repression Activation

T3

Coregulators

Corepressors

T3

RXR TR

RXR TR

TRE____TRE TRE____TRE

Figure 4.4 Mechanism of TR transcriptional control. Abbreviations : TR, thyroid hormone;
T3, triiodothyronine; RXR, retinoid X receptor; TRE, thyroid response element.
Source : From Moore and Guy (2005) .

dimerizes, and binds to the ER response element, thus inducing transcription of
estrogen target genes ( Kim et al., 2001; Ruh et al., 1999 ).
Upon entering the cell nucleus, the thyroid hormone (TH), especially in its trii-
odothyronine (T3) form, binds a TR isoform, changes its conformation, detaches
corepressors, and recruits coactivators ( Figure 4.4 ). The TR-RXR dimer binds to the
T3 response elements (TREs) in the promoters of TR-regulated genes and recruits
coactivators (enzymes of acetylation, deacetylation, phosphorylation, etc.), making
possible the expression of TH-regulated genes. In the absence of the TH, its nuclear
receptor (TR) forms a heterodimer with the retinoic X receptor (RXR) and in this