Regulation of Viral Transcription in Cells Infected with Iododeoxyuridine-substituted Simian Virus 40 as a Model for the Activation by Iododeoxyuridine of Latent Viral Genomes

5-Iododeoxyuridine (IdUrd) induces the expression of viruses in a variety of cell lines that harbor latent viral genomes. Moreover, IdUrd stimulates cellular as well as viral RNA synthesis in certain cells. In order to understand better the action of IdUrd on RNA metabolism, we have examined viral RNA synthesis in monkey cells infected with IdUrd-substituted simian virus 40 (SV40). Extensively substituted SV40, in which 18 to 35% of the thymidine residues were substituted by IdUrd, was 100-fold less viable (by plaque analysis) than was unsubstituted SV40, although the substituted virus induced 30 to 50% as much viral-specific RNA as did the unsubstituted virus. In contrast, SV40, containing only 10 to 15% IdUrd, substitution was almost as viable as unsubstituted virus, and the substituted SV40 induced 5-fold more viral-specific RNA, as well as longer viral messenger RNA transcripts, than did the unsubstituted virus. These results suggest that the lightly substituted, mutagenized SV40 genome may produce defective proteins which fail to regulate their own transcription. Cellular DNA into which halogenated pyrimidines have been incorporated may also induce the synthesis of defective regulatory proteins, including cellular repressors of transcription which normally maintain the latent state of integrated viral genomes.