Expression of early and late simian virus 40 transcripts in the absence of protein synthesis.

We examined the synthesis of early and late simian virus 40 (SV40) mRNA's in SV40-infected cells treated with two kinds of protein synthesis inhibitors. SV40 stimulated the synthesis of mRNA's for both large and small tumor antigens in cells pretreated with the drug emetine before the addition of virus. Emetine is a stringent inhibitor of protein synthesis and, thus, protein factors necessary for transcription and processing of these mRNA's probably preexist in the cell. Surprisingly, infection of cells pretreated with the protein synthesis inhibitor cycloheximide stimulated the synthesis of about 10-fold-higher levels of early viral mRNA's than did comparable infections of nontreated cells. This amplification of early viral mRNA steady-state levels is probably not due to inhibition of synthesis of the early A gene product since the same degree of drug-specific amplification was seen in SV40 tsA-infected cells that were cultured at the nonpermissive temperature. However, the most interesting effect of cycloheximide addition on viral mRNA synthesis was its stimulation of the appearance of late mRNA's in the cytoplasm of cells at early periods of infection. The synthesis of late mRNA's does not appear to require the A gene product as late RNAs can be found in the cytoplasm of cells infected with SV40 tsA mutants which have been maintained at 41 degrees C and continuously cultured in the presence of cycloheximide.