Transcription of mouse mammary tumor virus genes in tissues from high and low tumor incidence mouse strains

Abstract

Mouse mammary tumor virus-specific nucleotide sequences are present in the DNA of high and low tumor incidence mouse strains (Varmus et al., 1972). To examine regulation of these genes at the transcriptional level, we have measured and characterized the MMT-virus † † Abbreviations used: MMT virus, mouse mammary tumor virus; SS DNA, single-stranded DNA; C ot , product of DNA concentration and time; C rt product of RNA concentration and time. -specific RNA present in tumors, lactating mammary glands, spleens and livers from several mouse strains. Single-stranded DNA complementary to MMT virus 70 S RNA was synthesized by virus-associated DNA polymerase and hybridized to large excesses of unlabeled cell RNA. This SS DNA is highly specific for MMT virus sequences and copied from at least 60% of the genome. Complete hybridization of MMT virus SS DNA to tissue RNA was observed, using optimum annealing conditions (0·6 m-NaCl, 68 °C) and a stringent assay for hybridization (digestion of unhybridized DNA with a single strand-specific nuclease). Under these conditions, we can achieve reproducible measurement of the amount of virus-specific RNA per cell. Hybrid formation was further documented by density gradient centrifugation, and the character of the hybrids was assessed by thermal denaturation studies. Our principal findings include: 1. (1) large but variable amounts of virus-specific RNA (30 to 1000 genome equivalents per cell) in virus-producing tumors and lactating mammary glands from strains GR, Rill, C3H/an and DBA/2 mice; 2. (2) low amounts of MMT virus RNA (1·7 genome equivalents per cell) in virusfree mammary tumors from strain BALB/c mice; 3. (3) large quantities of MMT virus RNA (1300 genome equivalents per cell) in Leydig cell tumors containing incomplete virus particles in the cytoplasm; 4. (4) strain-dependent variation in the amount of virus-specific RNA (1 to 200 genome equivalents per cell) in lactating mammary glands from low tumor incidence strains (129, C68, C57BL/6, I and BALB/c); 5. (5) low levels of MMT virus-specific RNA (0.1 to 1 genome equivalent per cell) in livers and spleens from high and low incidence strains; 6. (6) correlation between virus production and amount of MMT virus RNA in the progeny of backcrosses between strains GR and C57 BL/6 mice; 7. (7) accurate base pairing, as judged by thermal stability, in hybrids between MMT virus SS DNA and RNA from virus-producing tissues, and minor degrees (3 to 5%) of base mismatching in hybrids formed between MMT virus DNA and RNA from virus-negative tissues. The results indicate that transcriptional controls are important in regulation of MMT virus gene expression and that hormonal and genetic factors may influence transcriptional control.