Hormonal response region in the mouse mammary tumor virus long terminal repeat can be dissociated from the proviral promoter and has enhancer properties.

The proviral DNA of mouse mammary tumor virus (MMTV) contains a regulatory region closely associated with its promoter, which subjects transcription to the control of glucocorticoid hormones. Delimitation analysis of a chimeric MMTV long terminal repeat-thymidine kinase gene (LTR-tk) has shown that the hormonal regulation sequence is confined to 202 nucleotides preceding the LTR-specific RNA initiation site. A second RNA initiation site (tk-specific mRNA) placed close to the regulatory MMTV sequence by in vitro recombination is also subjected to hormonal stimulation in transfected cells. A series of plasmids with deletions around the LTR cap site progressing from 3' to 5' was made and functionally tested. In vitro deletion of MMTV LTR sequences comprising the RNA initiation sequence and the "TATA" box do not effect hormonal regulation at the tk-specific mRNA start site. Nucleotides up to position -59 from the LTR initiation site could be deleted without influence on the glucocorticoid regulation, whereas deletions to position -65 abolished the hormonal effect on the tk gene transcription. A short MMTV LTR segment containing nucleotides -236 to -52 from the LTR initiation site was recombined with the tk gene or the alpha-globin gene. This fragment confers hormonal inducibility onto the heterologous genes over distances of 0.4 or 1.1 kilobases. The hormonal response region functions when it is placed either 5' or 3' of the regulated gene in both of the possible orientations and is reminiscent of an enhancer sequence.