THE VACCINIA VIRUS A18R PROTEIN PLAYS A ROLE IN VIRAL TRANSCRIPTION DURING BOTH THE EARLY AND THE LATE PHASES OF INFECTION

The vaccinia virus gene A18R is essential for virus infection. The los s of A18R protein function results in unregulated transcription late d uring virus infection from regions of the viral genome which are norma lly transcriptionally quiescent. We have characterized A18R protein ex pression in cells infected with wild-type virus and the A18R temperatu re-sensitive mutant Cts23. The A18R protein is expressed during early and late phases of infection. The A18R protein expressed by Cts23 viru s at permissive and nonpermissive temperatures is significantly less s table than the wild-type A18R protein. The A18R protein was identified as a virion component and localized by detergent extraction to the vi rion core. Virions purified from cells infected with the A18R temperat ure-sensitive mutants Cts-1, Cts22, and Cts23 are defective in early v iral transcription in vitro. The mutant transcription defect is not at tributable to gross defects in virion structure or virion DNA-dependen t RNA polymerase activity. We conclude that the A18R protein plays a r ole in viral transcription during the early phase of infection as well as during the late phase.