Abstract
IBT inhibits the formation of vaccinia virus progeny. IBT does not affect viral DNA replication or viral mRNA synthesis, or the functioning of "early" viral mRNA. However, in the presence of IBT the sedimentation coefficient of "late" viral mRNA, i.e., viral mRNA synthesized after the first 3 hr of the infection cycle, decreases from the normal value of 16S to about 8S within about 5 min of synthesis. As a result, the number of poly-ribosomes formed by viral mRNA is greatly reduced and viral proteins programed to be translated from then on are not formed.