Genetic studies of the ploidy of Moloney murine leukemia virus

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Abstract
An assay for Moloney murine leukemia virus was developed employing the production of morphologically altered foci in nonproducer mouse cells (15F) carrying murine sarcoma virus. Wild-type (wt) virus gave a ratio of titers at 39.degree. C/34.degree. C = 1.05 .+-. 0.45 (SD; n = 20). A spontaneous, thermosensitive (ts) mutant of Moloney murine leukemia virus, ts3, defective in a late viral function, gave 39.degree. C/34.degree. C = 0. A murine [mixed thymus and bone marrow] cell line (TB) was mixedly infected with ts3 and wt (multiplicities of infection, 7.8:4.3), cloned after infection and shown to be infected by both viruses. At 34.degree. C it produced wt, ts and particles of mixed parentage. The heterozygotes (hz) had ratios of assays 39.degree. C/34.degree. C = 0.06-0.84 (mean, 0.36). To eliminate possible interference by multiploid particles with determination of the proportions of the 3 types of particles, the virus produced by the mixedly infected, cloned cell line at 34.degree. C was distributed by velocity sedimentation in a sucrose gradient, and virus was picked from the lightest part of the gradient. The proportions of ts, wt and hz were 0.27, 0.26 and 0.47, respectively. Those particles identified as hz segregated ts, wt and hz in the proportions 0.24, 0.27 and 0.49, respectively. These values were not significantly different from those predicted from a diploid model of the genome.