Efficient infection of monkey cells with DNA of simian virus 40

With standard protocols for DNA infection, only a small fraction (about 4%) of monkey cells exposed to purified DNA of simian virus 40 (SV40) exhibits signs of infection. We have devised a protocol by which we can extend the time of exposure of BSC-1 cells to DNA in the presence of low concentrations of DEAE-dextran. The efficiency of infection is proportional to the time of exposure. With an 8-hr exposure, we are reproducibly able to infect 25% of the cells, and we have been able to achieve levels of infection as high as 50% with a 16-hr exposure. The percentage of cells infected was measured either by scoring for nuclei positive for SV40 tumor antigen or by an infectious centers assay. We also report the use of ethidium bromide as a nonspecific nuclear counterstain in the immunofluorescence assay for SV40 tumor antigen.