Metamorphic T3-response genes have specific co-regulator requirements.

Thyroid hormone receptors (TRs) have several regulatory functions in vertebrates. In the absence of thyroid hormone (T3; tri-iodothyronine), apo-TRs associate with co-repressors to repress transcription, whereas in the presence of T3, holo-TRs engage transcriptional coactivators. Although many studies have addressed the molecular mechanisms of T3 action, it is not known how specific physiological responses arise. We used T3-dependent amphibian metamorphosis to analyse how TRs interact with particular co-regulators to differentially regulate gene expression during development. Using chromatin immuno-precipitation to study tissue from pre-metamorphic tadpoles, we found that TRs are physically associated with T3-responsive promoters, whether or not T3 is present. Addition of T3 results in histone H4 acetylation specifically on T3-response genes. Most importantly, we show that individual T3-response genes have distinct co-regulator requirements, the T3-dependent co-repressor-to-coactivator switch being gene-specific for both co-regulator categories.