Regulation of Early and Late Simian Virus 40 Transcription: Overproduction of Early Viral RNA in the Absence of a Functional T-Antigen

Abstract

Virus-specific RNA synthesized in [African green] monkey [kidney] cells after infection by wild-type SV-40 and the early SV-40 temperature-sensitive mutant tsA58 was analyzed. The fraction of SV-40-specific RNA increased throughout infection with wild-type SV-40 or with tsA58 in direct proportion to the accumulation of progeny DNA molecules, suggesting their role in the late transcriptional process. Cytoplasmic fractions from cells infected at various temperatures (31.5.degree.-41.degree. C) by wild-type virus and harvested 48 h later contained 4-8% virus-specific RNA, of which 5-10% was early SV-40 RNA. Although 5-8% of the cytoplasmic RNA from tsA58-infected cells incubated at 31.5.degree.-37.degree. C for 48 h was virus specific, the percentage of early virus-specific RNA ranged from 25-80% as the incubation temperature increased. In tsa58-infected cultures incubated for 48 h at 41.degree. C (a temperature at which essentially no tsA58 DNA synthesis occurred), only 0.4% of the cytoplasmic RNA was virus specific, but at least 90% of this RNA was early. In experiments where cells were inoculated at 32.degree. C and shifted at 48 h postinfection to 40.degree. C for various times, the percentage of virus-specific pulse-labeled RNA varied from 3.5-10.0%. Of the virus-specific RNA, early SV-40 RNA ranged from 14-65% in tsA58-infected cultures. Analogous studies with Sarkosyl-extracted viral transcription complexes to incorporate label into nascent (unprocessed) viral RNA yielded essentially identical results. Apparently, the overproduction of early SV-40 RNA occurs at the level of synthesis. While cytosine arabinoside effectively terminated most viral DNA replication in wild-type-infected cells, the ratio of early to late viral RNA remained less than 1:9. The amount of virus-specific RNA synthesized depends directly on the amount of viral DNA available for use as templates; once viral DNA replication has occurred, presumably providing progeny SV-40 DNA molecules for templates, the level of transcription remains high. Termination of viral DNA replication does not terminate late SV-40 transcription. Early SV-40 RNA is overproduced by tsA58 at all temperatures, but especially at higher temperatures. Overproduction of early SV-40 RNA appears to be correlated with defectiveness of the tsA mutant T[tumor]-antigen. T-antigen may regulate its own production by repressing the synthesis of early viral RNA and/or by stimulating the synthesis of late SV-40 RNA.