The herpes simplex virus thymidine kinase gene promoter contains a novel thyroid hormone response element

Abstract

Thyroid hormone (T3) receptors (T3Rs) regulate transcription by binding to T3 response elements (TREs) located within promoter regions of T3-regulated genes. In rat pituitary GH4C1 cells, expression of a reporter containing herpes simplex virus thymidine kinase (TK) gene sequences (-105/+51) linked to the chloramphenicol acetyltransferase gene was stimulated 4- to 5-fold by T3. Linker scanning mutants of the TK promoter revealed that regions around -80 containing a CTF/NF-1 recognition sequence and around -10 are both required for regulation by T3. Endogenous T3Rs from GH4C1 cells labeled with [125I]T3 bound only to TK promoter DNA fragments containing the -10 region. The -22/-2 sequence (TK-TRE) contains half-sites oriented as an inverted repeat separated by 6 basepairs that are identical to and similar to an optimized TRE half-site. Purified chicken T3Rα1 forms apparent monomeric and dimeric complexes on the 32P-labeled TK-TRE, as found previously with an inverted repeat of the optimized TRE (TREp) with no basepair gap. T3 enhances the formation and alters the mobility of these complexes on both elements. When positioned up-stream of a heterologous promoter-chloramphenicol acetyltransferase reporter, the TK-TRE conferred T3 regulation by endogenous T3R in GH4C1 cells and by cotransfected chicken T3Rα1 in HeLa cells. The TK-TRE does not bind and is not activated P by retinoic acid receptor. T3Rs and nuclear proteins from GH4C1, HeLa, and COS1 cells form heterodimers on the TK-TRE which differ in abundance and mobility from heterodimers formed on the TREp. The identification of a TRE in the TK promoter raises the possibility that T3R or related proteins may play important roles in regulating the life cycle of herpes simplex virus.