Inhibition in the joining of DNA intermediates to growing simian virus 40 chains.

Viral DNA synthesis was inhibited for 1 h by the addition of 5-fluorodeoxyuridine (FUdR) to simian virus 40 (SV40)-infected cultures at 28 to 30 h postinfection. The subsequent addition of (3)H-thymidine to the inhibited cultures reverses the effect of the inhibitor, and during a 1-min labeling period there is rapid synthesis of SV40 DNA. By alkaline sedimentation analysis, it is observed that in FUdR-treated cultures there is synthesis of 4S SV40 DNA intermediates but there is a block in the joining of these intermediates to growing SV40 chain cultures. In addition to 4S fragments that are associated with replicating SV40 molecules, there is accumulation of SV40 DNA in the 6 to 8S region which is observed in neutral sucrose gradients. In an inhibited culture that is pulsed for 1 min with (3)H-thymidine and then chased for 10 min, accumulation of a Component II (Comp. II)-like material is observed. This Comp. II has the same neutral sedimentation characteristics and yields the same R(I) restriction endonuclease product as does authentic Comp. II. However, in alkali it is seen that it is composed of fragmented SV40 DNA. The basis for the failure of 4S fragments to join to growing SV40 chains is discussed. A model in which there is a requirement for two DNA polymerases and a ligase to permit SV40 DNA chain growth is proposed which is consistent with the data presented.