Differential expression of murine leukemia virus loci in chemically induced hybrid cells.

The time course of murine leukemia virus production after chemical induction was determined in hamster-mouse somatic cell hybrids containing the xenotropic murine leukemia virus induction locus Bxv-1 or the ecotropic locus Akv-2. By using these hybrids, induction could be studied in the absence of secondary virus spread because xenotropic viruses cannot infect hybrid cells and ecotropic viruses cannot infect hybrids which have lost mouse chromosome 5. After induction, hybrids with Bxv-1 produced only a transient burst of virus, whereas those with Akv-2 continued to produce virus for periods in excess of 3 months. The presence or absence of other mouse chromosomes in the hybrid lines did not alter these induction patterns. Thus, endogenous murine leukemia virus loci differ in their response to induction, and both inducibility and the kinetics of virus expression are controlled at or near these proviral loci.