Temperature-sensitive Events in the Growth of Alastrim Virus in Chick Embryo Cells

The growth of alastrim virus in chick embryo cells was studied under one-step conditions at various temperatures. Limited growth was found at 37 °C but there was none at 38 °C or above. Temperature-shift experiments suggested that at 38 °C only late events were temperature-sensitive. Virus DNA synthesis, induction of early enzymes and the production of early antigens were all substantially normal at 38 °C. In a study of late events at 38 °C, particle formation was found to be almost completely inhibited. Although a few immature particles were seen by electron microscopy of thin sections, cytoplasmic DNA labelled with [3H]-thymidine did not become resistant to DNase and particles containing DNA were not seen after centrifuging on sucrose density gradients. There was no late rise in DNA-dependent RNA polymerase activity. Late antigen production at 38 °C appeared normal in agar gel diffusion studies both in the time of appearance and in the number of lines present, but the LS-antigen complex was slightly reduced in amount. Production of haemagglutinin was completely suppressed at 38 °C. It is concluded that the major effect of temperature on the growth of alastrim virus is to inhibit a very early stage in particle formation.