POSITIVE AND NEGATIVE MODULATION OF JUN ACTION BY THYROID-HORMONE RECEPTOR AT A UNIQUE AP1 SITE

We have characterized the putative AP1 site in the backbone of pUC pla smids and found unique regulatory effects. The site, which mapped to a 19-bp region around nucleotide 37, conferred transcriptional activati on by Jun or Jun/Fos that was boosted up to fivefold by unliganded thy roid hormone receptor (TR). Thyroid hormone changed potentiation of th e Jun response by TR into repression. Although the plasmid sequence is a near-perfect consensus AP1 site, the perfect consensus AP1 site fro m the human collagenase promoter did not show the same effects. Deleti on of the ligand binding domain of the TR eliminated the ability of th e receptor to boost Jun activity, and deletion, mutation, or changes i n specificity of the DNA binding domain eliminated both its ability to potentiate Jun activity and repress with hormone. In vitro Jun/Fos co mplexes bound the operative plasmid fragment, and the presence of TR i nterfered very little with Jun/Fos binding activity. Protein interacti on studies in the absence of DNA showed that TR bound Jun protein in s olution either in the presence or in the absence of hormone. These obs ervations suggest a mechanism for synergy and repression by TR through modulation of Jun activity: positive when TR is unliganded, and negat ive when hormone is bound. They also suggest that the presence of the plasmid element can confound studies of the regulation of linked promo ters.