Characterization of simian virus 40 tsA58 transcriptional intermediates at restrictive temperatures: relationship between DNA replication and transcription

Abstract

When nuclei from simian virus 40 (SV40)-infected cells are lysed with Sarkosyl and the chromatin is pelleted, the supernatant fluid contains a nucleoprotein complex capable of synthesizing viral RNA (Laub and Aloni, Virology 75:346-354, 1976; Gariglio and Mousset, FEBS Lett. 56:149-155, 1975). The level of activity of the RNA polymerase in the complex increased during infection in parallel with the amount of viral DNA that had been synthesized. If cells infected at 33 degrees C with the SV40 mutant tsA 58 were shifted to the nonpermissive temperature of 40 degrees C at any time between 18 and 48 h postinfection, no viral DNA replication was detected after 45 min and no new rounds of synthesis were initiated after 20 to 30 min. However, after this shift, polymerase activity associated with the nucleo-protein complex did continue to increase for 5 h, at which time it reached a plateau. There was an increase of RNA synthesized from both the early (E) and late (L) SV40 DNA strands, and there was a threefold increase in the ratio of early-to-late RNA species after the shift. In comparable experiments with cells infected with wild-type virions, no increase in polymerase activity occurred because of the temperature change alone. At 33 degrees C, the relative amount of RNA transcribed from the wild-type E-strand was less than tsA 58 at 33 degrees C and did not increase after a shift to 40 degrees C. The tsA 58 transcriptional complexes extracted from cells grown at 33 degrees C sedimented heterogeneously in sucrose gradients, with a peak near 26S. There were no detectable alterations in the sedimentation properties of the complexes when tsA 58-infected cells were shifted to 40 degrees C for 2 h. We conclude that continued synthesis of viral DNA is not an obligatory prerequisite for maintenance of late viral transcription nor is the sedimentation of the transcriptional complex at 26S related to actively replicating DNA molecules serving as templates for transcription. Further, an increase in late transcription can occur under conditions where reinitiation of viral DNA synthesis is prevented. The increase in the synthesis of early and late RNA at the restrictive temperature without concurrent DNA synthesis is discussed in relationship to the function of the A gene product.