Mouse mammary tumor virus gene expression regulated in trans by Lps locus

Expression of mouse mammary tumor virus (MMTV) in the lactating mammary glands of uninfected mice varies between strains of mice in a manner largely independent of the proviral content. Previous linkage analysis in the mouse suggested that the Lps locus was associated with steady-state levels of MMTV RNA. The Lps locus mediates the mouse's response to the injection of lipopolysaccharide (LPS) in the responder mouse while mice with the deficient allele are incapable of responding. Injecting LPS-responder mice, C3HfB/HeN, and nonresponder mice, C3Hf/HeJ, with LPS resulted in a threefold increase in the level of MMTV RNA in responder mice but had no effect on nonresponders. The increased level was due to only one of the possible MMTV transcripts: the 1.7-kb transcript containing the open reading frame (orf) of the long terminal repeat (LTR). The level of MMTV-specific transcripts, then, is regulated by the Lps locus, a cellular gene which is not linked to any viral coding sequences and therefore must act in trans.