GLUCOCORTICOID REGULATION OF A TRANSCRIPTION FACTOR THAT BINDS AN INITIATOR-LIKE ELEMENT IN THE MURINE THYMIDINE KINASE (TK-1) PROMOTER

Glucocorticoids inhibit transcription of the murine cytoplasmic thymid ine kinase gene (Tk-1). Glucocorticoid regulation of Tk-1 transcriptio n can be demonstrated in cells that are arrested in late G1. This obse rvation indicates that inhibition of Tk-l expression is not dependent upon redistribution within the cell cycle but is due to glucocorticoid regulation of this gene. Transfection studies have been carried out u sing chimeric genes in which restriction fragments of the Tk-1 promote r were fused to chloramphenicol acetyltransferase or neomycin phosphot ransferase. These chimeric reporters were assayed for stable expressio n and glucocorticoid regulation in P1798 lymphoma cells. A 140-bp frag ment, extending from -143 to -3 bp with respect to the thymidine kinas e translational start site, was capable of both basal and glucocortico id-regulated transcription of reporter genes. The extent of inhibition by glucocorticoids was similar to that observed for the endogenous ge ne, and no increase in basal expression or the extent of inhibition wa s observed with constructs containing additional 5'-flanking DNA. The 140-bp Tk-1 core promoter fragment binds to transcription factors in e xtracts from P1798 cells. Control cell extracts contain factors that b ind to and protect (from deoxyribonuclease I) a distal promoter elemen t from -106 to -87 bp, relative to the translational start site. A sec ond, proximal element was protected at -43 to -36 bp. The proximal ele ment of the Tk-1 promoter resembles an RNA polymerase II initiator ele ment. No other elements were protected. Glucocorticoids inhibit the am ount or activity of the transcription factor that binds to this initia tor-like element within the Tk-1 promoter. This element, when fused to upstream activation sequences from the herpes simplex virus thymidine kinase promoter, conveys glucocorticoid sensitivity in cis.