A ROLE FOR NUCLEOSOME ASSEMBLY IN BOTH SILENCING AND ACTIVATION OF THE XENOPUS TR-BETA-A GENE BY THE THYROID-HORMONE RECEPTOR

We have assembled the thyroid hormone-inducible promoter of the Xenopu s thyroid hormone receptor (TR)beta A gene into chromatin using replic ation-coupled and -independent assembly pathways in vivo. We establish that heterodimers of TR and 9-cis retinoic acid receptors (RXR) can b ind to their recognition sites within chromatin both in vivo and in vi tro and alternately repress or activate transcription dependent on the absence or presence of thyroid hormone. Maximal transcriptional repre ssion requires the presence of unliganded TR/RXR heterodimers during r eplication-coupled chromatin assembly. We demonstrate an increase in t ranscription directed by the TR beta A promoter of over two orders of magnitude in vivo, following the addition of thyroid hormone. This inc rease in transcription involves the relief of the repressed state that is established by the unliganded TR/RXR heterodimer during replicatio n-coupled chromatin assembly. The association of thyroid hormone with the chromatin-bound TR/RXR heterodimer leads to the disruption of loca l chromatin structure in a transcription-independent process. Thus, ch romatin structure has multiple roles in the regulation of TR beta A ge ne expression in vivo: The TR/RXR heterodimer recognizes the response element within chromatin, TR/RXR makes use of the chromatin assembly p rocess to silence transcription more efficiently, and TR/RXR directs t he disruption of local chromatin structure in response to thyroid horm one.