Rapid induction of rat liver S14 gene transcription by thyroid hormone.

The mRNA coding for the rat liver S14 protein (Mr 17,000; pI 4.9) is rapidly induced by triiodothyronine (T3) (Jump, D. B., Narayan, P., Towle, H. C., and Oppenheimer, J. H. (1984) J. Biol. Chem. 259, 2789-2797). In an effort to define the molecular basis for the rapid increase in mRNAS14, the in vitro run-on activity and chromatin structure of the S14 gene was examined. Following injection of hypothyroid rats with a receptor-saturating dose of T3, a 5-min lag period preceded a rapid increase in S14 gene transcription. S14 transcriptional activity was induced 3.8-fold within 15 min and reached nearly 70% of the maximal 9-fold induction within 60 min of T3 administration. Hepatic mRNAS14 levels were induced 2.4-, 19-, and 24-fold within 15 min, 4 h, and 24 h, respectively. Thus, the rapid T3-mediated induction of mRNAS14 was due, in large part, to a rapid and apparent direct effect of T3 on S14 transcriptional activity. Analysis of S14 chromatin structure showed that the Hss-3 DNase I hypersensitive site located 3.3 kilobases (kb) upstream from the hepatic S14 cap site was present in both euthyroid and hyperthyroid states, but either absent or present at diminished DNase I sensitivity in the hypothyroid state. However, within 5 min of T3 administration to hypothyroid rats, the Hss-3 DNase I hypersensitive site was significantly induced. The induction of this structure preceded T3 induction of S14 gene transcription. Other DNase I hypersensitive sites located adjacent to the S14 cap site at -65 to -265 base pairs (Hss-1) or upstream at -1.3 kb (Hss-2), -2.1 kb (Hss-3'), -5.3 kb (Hss-4), and -6.2 kb (Hss-5) remained unaffected by changes in S14 gene transcription. The rapid T3 effect on the Hss-3 DNase I hypersensitive site may reflect the presence of T3 receptors in the vicinity of this chromatin locus. Modification of chromatin structure in the vicinity of the Hss-3 site may be an important antecedent event for T3-mediated induction of S14 gene transcription.