Structural and nonstructural proteins of strain Colburn cytomegalovirus

The growth of most Rous sarcoma viruses (RSV) is severely restricted on MSB-1 cells (a line of chicken T lymphoblasts) in comparison to growth on chicken embryo fibroblast (CEF). Nonconditional transformation defective mutants of RSV from which the complete src region has been deleted (td RSV) are not subject to growth restriction. We examined the formation and integration of RSV and td RSV in MSB-1 cells following high multiplicity infection. Nearly equivalent quantities of the linear form of unintegrated RSV and td RSV DNA were formed in these cells during the first 10 hr after infection. Linear RSV DNA from MSB-1 cells could not be distinguished from linear RSV from CEF by restriction endonuclease analysis and by previously described transfection assays (P. E. Neiman, C. McMillin-Helsel, and G. M. Cooper, 1978, Virology 89,360–371). Beyond 10 hr after infection, and with progressive cell growth in the MSB-1 cultures, the level of RSV linear DNA rapidly decreased. Presumptive circular RSV DNA was detected only transiently, and at very low levels, about 15 hr after infection. Association of RSV DNA with high-molecular-weight chromosomal DNA, i.e., integration, was not detected in this study. In contrast, nearly constant levels of td RSV unintegrated linear DNA and, after 20 hr, circular DNA persisted in MSB-1 cells for at least 7 days after infection. Integration of td RSV proviral DNA was inefficient, occurring in only about 5% of MSB-1 cells (even at very high multiplicities of infection) in the first round of infection, and in 25–40% of cells by 3 days after infection. Almost all MSB-1 cells containing td RSV DNA produced virus. Analysis of eight nonconditional transformation defective mutants of RSV which retain the src region to different extents showed that all of these mutants replicated to the same normal titer on MSB-1 cells as on CEF without further deletion of the src region. Two temperature sensitive src mutants that thermal inactivation of the scr gene on MSB-1 cells at both 35° and 41°, indicating that thermal inactivation of the src gene product could not abrogate the replication block. These studies clearly demonstrate that the presence of the src region in RSV impedes the formation and/or integration of provirus in some types of host cells.