Superinfection by strains of tobacco streak virus

Synthesis of vaccinia virus DNA was studied by following the incorporation of [³H]thymidine in the cytoplasm of infected cells. During short pulses (1–5 min) the radioactive precursor was incorporated into single-stranded DNA fragments that sedimented as 10–12 S in neutral sucrose gradients. After the chase, these fragments were converted into a 34 S intermediate before becoming mature size molecules (68 S). Following longer periods of chase, the labeled DNA sedimented as cross-linked molecules in alkaline sucrose gradients (91–106 S) suggesting that the formation of cross-links is a postreplicative process. The presence of RNA covalently linked to DNA in the 12 S fragments was suggested by a shift in the equilibrium density sedimentation and by sensitivity to spleen phosphodiesterase after alkaline hydrolysis. Determination of the molecular weight of vaccinia DNA synthesized in cultures containing bromodeoxyuridine (BrUdR) demonstrated that replication proceeds in two directions.