Irreversible effects of cycloheximide during the early period of vaccinia virus replicaon.

The presence of cycloheximide, an inhibitor of protein synthesis, during the period 30 to 60 min after vaccinia infection produced an irreversible block in virus replication. In contrast (i) cycloheximide given at earlier or later times, even for prolonged periods, did not prevent continuation of the infectious cycle after removal of the drug, and (ii) treatment with cycloheximide during the first 2 hr did not prevent virus growth when the early stages of replication proceeded more slowly due to infection with a low multiplicity of virus. These findings were interpreted as an indication that protein synthesis is required at a critical time in the virus growth cycle. Under the conditions in which brief cycloheximide treatment prevented virus growth, ribonucleic acid (RNA) synthesis continued at an undiminished rate for at least 2 hr after removal of the drug. Although this RNA appeared identical by polyacrylamide gel electrophoresis to "early" viral messenger RNA, it was not found associated with ribosomes or polyribosomes. Failure to observe viral protein synthesis was consistent with the latter finding. It appeared unlikely that the translational block resulted from inadequate removal of cycloheximide, since the effects of the drug were shown to be reversible at earlier or later times in infection or even at the same time when a lower multiplicity of virus was used. Interference with the normal synthesis of specific viral protein factors required for translation was postulated to explain the results.