Distinct sequence elements involved in the glucocorticoid regulation of the mouse mammary tumor virus promoter identified by linker scanning mutagenesis

Abstract

In the proviral DNA of mouse mammary tumor virus (MMTV), sequences up to ≈ 200 base-pairs from the RNA start site are required for stimulation of transcription by glucocorticoid hormones in cultured cells. A total of 26 mutant plasmids with clustered point mutations or small deletions in the hormone control region of the MMTV long terminal repeat were constructed, linked to the coding portion of the Herpes simplex virus thymidine kinase gene, and introduced by transfection into LTK − cells. Transcription from mutant DNA in the presence or absence of hormone was quantified by S 1 nuclease protection assays. Our analysis revealed the presence of at least three control elements that affect the extent of transcription stimulation by glucocorticoid hormones: (1) a distal element, between − 181 and − 172 base-pairs from the RNA initiation site. Linker scanning mutants in this segment have a reduction of up to 20-fold in the hormone response with respect to wild type. (2) An element around position − 120, defined by a mutation of 4 base-pairs between − 121 and − 117, which causes a fivefold reduction. (3) An element from ≈ − 78 to − 70, defined by a mutant with also a roughly fivefold lower stimulation. The first two are included in areas that have been shown by others to interact in vitro with hormone-receptor complexes; the last one overlaps the in vitro binding site of a nuclear protein factor. A mutant lacking all three elements (− 193 to − 70) is completely non-inducible by glucocorticoids. Together with earlier results obtained with 5′ deletion mutants, the data show that the largest contribution to the stimulatory response is made by the distal element, which however does require the presence of both more-proximal ones for the response to be maximal. In the absence of the distal one, the two proximal elements together produce a residual stimulation in the order of 5 to 10% of wild type, while the − 70 element alone is ineffective. In addition, we show that a functional TATA homology is required for maximum stimulation. It appears that transcriptional regulation of MMTV by glucocorticoid hormones is achieved by the concerted action of multiple sequence modules, not all of which correspond to receptor binding sites in vitro.