Integration and Activity of Mammary Tumor Virus Genes: Regulation by Hormone Receptors and Chromosomal Position

Abstract

Little is known about the mechanisms by which mammary tumor virus (MTV) or other retrovirus DNAs integrate into host genomes; it is intriguing, however, that the integrated proviruses are organized in a fashion strikingly reminiscent of bacterial transposons and certain movable genetic elements in eukaryotes (Calos and Miller 1980; Shimotohno et al. 1980; J. E. Majors and H. E. Varmus, in prep.). At the termini of MTV, for example, are direct-repeat sequences (termed long terminal repeats or LTR) of 1.3 kilobase pairs (kbp), which themselves contain 6-bp inverted-repeat sequences at their termini (J. E. Majors and H. E. Varmus, in prep.). MTV proviruses integrate as intact elements at many loci within the host DNA (Ringold et al. 1979); as with classical movable elements, MTV insertion is accompanied by duplication of several base pairs of host DNA at the sites of integration (J. E. Majors and H. E. Varmus, in prep.)...