Transcription of simian virus 40. V. Regulattion of simian virus 40 gene expression.

RNA "exhaustion type" hybridization was used to measure the complementarity of nuclear and cytoplasmic viral RNA to the early (E) and late (L) simian virus 40 (SV40) DNA strands. This type of hybridization measures the amount of labeled RNA complementary to each of the two DNA strands, rather than the fraction of each SV40 DNA strand that is homologous to SV40 RNA. At 48 h after infection, about 5% of the nuclear newly synthesized viral RNA was complementary to the E-strand (- strand) and 95% was complementary to the L-strand (+ strand). This proportion was independent of the labeling time, indicating similar accumulation of the E- and L-RNA transcripts in the nucleus. The nuclear E- and L-viral RNA transcripts sedimented in a similar manner on sucrose gradients. Of the cytoplasmic viral RNA only about 1% was complementary to the E-strand, these molecules sedimenting at 19S, whereas 99% were complementary to the L-strand and sedimented at 19S and 16S. The abundance of E-RNA transcripts in nuclei of cells infected with serially passaged virus was about four times higher than that in nuclei of cells infected with plaque-purified virus; however, the size and proportion of the corresponding cytoplasmic E- and L-RNA transcripts was independent of the type of virus used to infect the cells. According to these results at least two control mechanisms regulate viral gene expression in productively infected cells, one operates at the trnascriptional level and the second at the post-transcriptional level.