Trithorax-group proteins

Trithorax-group proteins are chromatin regulatory proteins which generally act to maintain gene expression. This group of proteins is rather heterogeneous and contains members that form several multiprotein chromatin complexes. One main activity of the Trithorax complex is to activate transcription by inducing trimethylation of lysine 4 of histone H3 (H3K4me3) at specific regulatory sites in their target chromatin. This active state is reinforced by acetylation of histone H4. In addition, some Trithorax-group proteins have chromatin remodeling activity, using the energy of ATP to mobilize nucleosomes.

Polycomb-group proteins are the repressive couterpart to the Trithorax group proteins in that they also bind to the chromatin at the Hox genes and act to induce trimethylation of lysine 27 of histone H3 (H3K27me3), to prevent acetylation at histone H4, and to counteract nuclesome remodeling, thus maintaining silent states of gene expression.