Cloned endogenous mouse mammary tumor virus DNA is biologically active in transfected mouse cells and its expression is stimulated by glucocorticoid hormones

We have isolated an intact copy of endogenous mouse mammary tumor virus (MMTV) DNA with its adjacent cellular sequences from a gene library of A/J mouse parotid DNA. The bacteriophage λ DNA containing the viral genome was introduced into Ltk − mouse cells together with a plasmid containing the thymidine kinase ( tk) gene of herpes simplex virus. Tk + L cells containing the cloned MMTV DNA-synthesized virus-specific RNA and proteins. In all except one cell clone the viral gene expression was stimulated by the addition of dexamethasone. The degree of methylation of the active transfected viral DNA was much lower than that observed for the inactive resident endogenous proviruses present in untransfected and transfected mouse L cells.