Acquisition of sequences homologous to host deoxyribonucleic acid by closed circular simian virus 40 deoxyribonucleic acid.

The synthesis of closed circular simian virus 40 (SV40) deoxyribonucleic acid (DNA) containing sequences homologous to host cell DNA depends upon the conditions under which the cells are infected. When BS-C-1 monkey cells were infected with non-plaque-purified virus at low multiplicity of infection [MOI, 0.032 plaque-forming units (PFU)/cell], little, if any, of the SV40 DNA extracted from the infected cells hybridized to host DNA; but when increasingly higher multiplicities were used (in the range 0.16 to 3,000 PFU/cell), an increasingly greater amount of the extracted SV40 DNA hybridized to host DNA. The same effect was observed when the closed circular SV40 DNA was extracted from purified virions (grown at low and high MOI) rather than from the infected cell complex. When the cells were infected at high MOI with plaque-purified virus (11 viral clones were tested), none of the SV40 DNA extracted from the cells hybridized detectably with host cell DNA. However, plaque-purified virus that was serially passaged, undiluted, induced the synthesis of virus DNA which again showed extensive homology to host DNA. It is suggested that, under certain circumstances, recombination occurs between viral and host DNA during lytic infection which results in the incorporation of host DNA sequences into closed circular SV40 DNA.