Characterization of a temperature-sensitive mutant of vaccinia virus reveals a novel function that prevents virus-induced breakdown of RNA.

We have attempted to characterize the molecular defect in a temperature-sensitive mutant of vaccinia virus, ts22, which has an abortive late phenotype. At the nonpermissive temperature, ts22 displays normal viral protein synthesis until 8 h postinfection. Between 8 and 10 h after infection all viral protein synthesis ceases abruptly. Characterization of ts22 revealed that (i) primary transcription of late viral genes was not grossly impaired, (ii) late viral mRNA was biologically inactive since it could not stimulate in vitro protein synthesis, and (iii) extensive cleavage of rRNA and late viral mRNA occurred at the time that viral protein synthesis aborted in vivo. These data suggest that ts22 is defective in a function which prevents host rRNA and viral mRNA from being degraded. Inhibitor studies with cytosine arabinoside and cycloheximide showed that induction of and protection from rRNA breakdown occurred at approximately the same time during infection and required late viral gene expression. The viral protein synthesis pattern observed in vaccinia virus-infected cells treated with the drug isatin-beta-thiosemicarbazone was strikingly similar to that observed in ts22-infected cells at the nonpermissive temperature (J. Cooper, B. Moss, and E. Katz, Virology 96:381-392, 1979). Analysis of rRNA integrity in isatin-beta-thiosemicarbazone-treated, vaccinia virus-infected cells revealed extensive cleavage of rRNA, suggesting that the ts22 and drug inhibitor may function in the same pathway.