Mouse mammary tumor virus genes: regulation of expression by glucocorticoids and structural analysis with restriction endonucleases.

The production of mouse mammary tumor virus (MTV) is stimulated by treatment of mammary carcinoma cells with glucocorticoid hormones (e.g., dexamethasone). The reaction appears to be a primary and receptor-mediated phenomenon in which the rate of synthesis of MTV RNA is selectively increased; the maximally induced rate is reached within 15 min after addition of the hormone. Production of MTV RNA is also stimulated by dexamethasone in some but not all clonal isolates of nonmurine cells infected by the virus. Using restriction endonucleases, we have characterized both integrated and unintegrated viral DNA from clones of infected rat hepatoma (HTC) cells. We suggest that the site(s) in the rat cell genome into which MTV DNA integrates may be a determining factor for MTV RNA synthesis.