Quantitative studies of integration of murine leukemia virus after exogenous infection.

Using a [3H]DNA probe prepared from AKR murine leukemia virus, we determined the number of copies of the AKR virus genome integrated into the cellular DNA after exogenous infection of NIH mouse, AKR mouse, and rat cells in tissue culture. NIH mouse cells, which lack a portion of the viral genome (referred to as Gross-AKR specific sequences), incorporated three to four copies of these sequences per haploid genome. AKR cells, in which the Gross-AKR specific sequences are already present as three to four copies per haploid genome, did not shwo any distinct change in copy number after infection. Rat cells, which lack DNA sequences homologous to murine leukemia virus, incorporated one copy of the viral genome per haploid genome. It is inferred that the presence of viral sequences may affect the efficiency of integration of exogenous provirus, and that there may be a limit to the number of copies that can be inserted.