Effect of 5-bromodeoxyuridine and actinomycin D on the growth of visna virus in cell cultures

Bromodeoxyuridine (BUDR) and actinomycin D were found to inhibit the growth of visna virus in monolayer cultures of fibroblast-like sheep cells. The inhibitory effect of BUDR was maximal at a concentration of 33 μM. The inhibition was reversed by thymidine and to some extent by deoxyuridine and deoxycytidine. A complete reversal was not obtained even by adding a large excess of thymidine. BUDR was most effective as inhibitor when added to the cell cultures 1-2 hours after infection with virus. The effect decreased rapidly with time and was hardly detectable when BUDR was added 8 hours after infection. The effect of BUDR preceded the formation of progeny virus by 15-20 hours. Virus multiplication was inhibited by 0.25 μg per milliliter of actinomycin applied to infected cultures for a period of 2 1 2 hours. The inhibition was complete even when actinomycin was added to the cell cultures as late as 24 hours after infection, when formation of progeny virus had begun. The results are discussed with regard to the role of nucleic acids in the formation of visna virus.