No abstract

Molecular clones containing the 3' half of newly integrated mouse mammary tumor virus (MMTV) DNA with adjacent mouse cellular sequences were characterized. In addition, we cloned the long terminal redundancy joint from the unintegrated circular form of MMTV DNA. The entire nucleotide sequence of the integrated and part of the unintegrated terminal redundancy was determined; this allowed us to delineate the boundaries of the MMTV long terminal redundancy, which comprises 1,327 base pairs. The position of possible RNA polymerase II initiation and termination signals corresponded closely to the expected regions of viral RNA initiation and termination specified by current models. The MMTV long terminal redundancy also contained a large open reading frame with sufficient information for a protein of 198 amino acids. Initial comparison of flanking 3' cellular sequences from three independent integrated clones suggested there was no host sequence specificity in the MMTV integration event. However, specificity of integration with respect to viral sequences was precise.

Addition of the transcriptional enhancers present in the U3 region of the Harvey murine sarcoma virus (HaMuSV) long terminal repeat (LTR) to recombinant chimeras in which the HaMuSV transforming gene (Ha‐v‐ras) is expressed from the mouse mammary tumor virus (MMTV) promoter increases the ability of the MMTV v‐ras chimeras to transform mouse fibroblasts in culture 50‐ to 100‐fold. Significant stimulation of transfection efficiency occurs only when glucocorticoids are present in the culture medium. Glucocorticoids also elevate the steady‐state concentration of MMTV‐initiated v‐ras mRNA in cell lines isolated from these transfections, and MMTV‐v‐ras fusion transcripts are initiated at the normal MMTV cap site; potential cryptic initiation events associated with the enhancer could not be detected. The ability of the enhancer to increase the transcriptional activity of the MMTV promoter was also studied in acute transfection assays where expression of the chloramphenical acetyl transferase (CAT) gene is driven by the MMTV promoter. In this system the strong positive effect on MMTV transcription is again obtained only when the cells are hormone treated. These experiments indicate that the hormone‐regulatory region is capable of modulating the function of an exogenously introduced enhancer element.