Specific targeting and constitutive association of histone deacetylase complexes during transcriptional repression

Specific recruitment of corepressor complexes containing histone deacetylases (HDAC) by transcription factors is believed to play an essential role in transcriptional repression. Recent studies indicate that repression by unliganded nuclear hormone receptors and by the Mad family of repressors requires distinct HDAC-containing corepressor complexes. In this work, we show that unliganded TR specifically recruits only the closely related N-CoR and SMRT-HDAC3 complexes, whereas the Mad1 recruits only the Sin3-HDAC1/2 complex. Significantly, both the Sin3 and Mi-2/NURD complexes also exhibit constitutive association with chromatin and contribute to chromatin deacetylation in a nontargeted fashion. These results suggest that HDAC complexes can contribute to gene repression by two distinct mechanisms as follows: (1) specific targeting by repressors and (2) constitutive association with chromatin.