Transcriptional repression of a hormone-responsive promoter

The activity of the mouse mammary tumor virus promoter was assessed in various sequence contexts with a transient transfection assay in which promoter activity was determined by way of expression of a linked gene encoding chloramphenicol acetyltransferase, as well as by direct analysis of RNA transcripts. The results indicate that the proviral long terminal repeat contains a negative transcriptional control element in addition to the glucocorticoid-responsive transcriptional enhancer that has been described previously. The negative element is able to function in both orientations and, at least to some extent, at multiple positions with respect to the regulated transcription unit. The effects on gene expression cannot be explained by alterations in transfection efficiency. The element has been localized to a 91 base pair fragment located immediately 5' of binding sites for the glucocorticoid receptor protein that have been defined in vitro. The role of the negative element may be to repress the inherent activity of the proviral promoter in the absence of glucocorticoids, resulting in an increased ratio of gene expression in the presence and absence of hormone.