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Harvey and Kirsten murine sarcoma viruses have previously been shown to transform fibroblastic cells in culture, and type C virus pseudotypes of these viruses cause erythroleukemia in susceptible mice. We report a cell culture assay for quantitating the growth-promoting effect of Harvey and Kirsten viruses on erythroid cells. Murine hemopoietic cells were infected in vitro with Harvey or Kirsten sarcoma virus, and then cultured in methylcellulose in the presence of relatively low concentrations of erythropoietin. Under these conditions, large colonies of erythroid cells form in the semi-solid culture media 6 to 8 days after infection. The induction of erythroid bursts was not caused by the murine type C helper viruses used to pseudotype either Ha-MuSV or Ki-MuSV, or by media from cells carrying the Ki-MuSV and Ha-MuSV genomes. Induction of the erythroid colonies is under genetic control at the Fv1 susceptibility locus, but not at the Fv2 susceptibility locus. A striking feature of the erythroid colonies induced by the Harvey and Kirsten viruses was that they not only proliferated to large size but also differentiated along the erythroid lineage and synthesized hemoglobin. The results indicate that Ha-MuSV and Ki-MuSV can induce proliferation of erythroid precursor cells apparently without interfering with the differentiation program of the cells. The relation between the growth-promotion effect of these viruses on erythroid precursor cells and their ability to induce erythroleukemia is discussed.

The mammary tumor induction genes Mtv-1 in mouse strain DBAf and Mtv-2 in strain GR control the complete expression of the endogenous mouse mammary tumor virus (MMTV). We have used a combination of genetic, biochemical and molecular biological methods to identify and correlate specific copies of the endogenous MMTV proviral genes with the biological properties of the tumor induction genes Mtv-1 and Mtv-2. These Mtv induction genes contain specific MMTV proviral information, as was concluded from restriction enzyme analysis and molecular hybridization of DNAs of congenic mouse strains and of progenitors of backcross populations. The congenic strains differed from the parental strains GR and 020 only in the Mtv-2 gene, one lacking the Mtv-2 gene (GR/Mtv-2-) and one having obtained this gene (020/Mtv-2+). The gain or loss coincided with two Eco RI cellular DNA fragments containing MMTV DNA information. Since Eco RI cuts the exogenous proviral variant of MMTV DNA once, we assume that these two cellular DNA fragments contain one MMTV provirus. The same cellular DNA fragments containing MMTV DNA information segregated together with MMTV expression in the offspring population of the backcross. In a similar backcross analysis of the induction gene Mtv-1 it was also demonstrated that the Mtv-1 gene comprises one MMTV provirus. These data indicate that Mtv induction genes contain specific but different MMTV proviral genes and that nly a limited number of the MMTV proviruses present in the cellular DNA is associated with the control of proviral expression.

RF/J mice carry three endogenous ecotropic murine leukemia proviruses designated Emv-1, Emv-16, and Emv-17. Two of these proviruses, Emv-16 and Emv-17, are tightly linked and segregate with the high viremia phenotype in backcrossed mice. During the derivation of an SWR/J strain congenic for Emv-16 and Emv-17, we found that many of the progeny derived from female virus carriers acquired new germline ecotropic proviruses. Additional genetic crosses suggested that these proviruses are acquired early in development by virus infection and that this strain combination is particularly susceptible to these events. The frequency of proviral acquisition was only about 10-fold lower than the frequency of P element acquisition in dysgenic crosses of Drosophila melanogaster. Since virus integration in these hybrids occurs at many different sites, these types of hybrids may ultimately be useful for generating virally induced mutations that are amenable to study at the molecular level.

Previous genetic and biochemical studies identified three large RNAase T1-resistant oligonucleotides, each associated with either the N-, B- or NB-tropism of murine C-type viruses of BALB/c origin. These oligonucleotides were shown to lie in the 5' third of the oligonucleotide maps of their respective viruses. We sequenced the three oligonucleotides and found that they share a 10 base sequence. Together these observations provide good evidence that the determinants of N-, B- or NB-tropism monitored by the three oligonucleotides are allelic. The oligonucleotides associated with N- and B-tropism differ in sequence at four of sixteen nucleotides, while the B- and NB-tropism-associated oligonucleotides differ in sequence by only one base out of sixteen. These results are consistent with the possibilities that B-tropic viruses may arise from N-tropic viruses by recombination, while NB-tropic viruses may arise from B-tropic virus by mutation. An unexplained finding was that a 10 base sequence present in the oligonucleotide associated with N-tropism is also found in the 3' third of the genomes of the N-, B- and NB-tropic viruses studied.