The multiplication of variola virus in HeLa cells was examined under one-step growth conditions at 35° and at 40°. At 35°, after an eclipse of 10 hr, exponential growth proceeded for 10 to 12 hr with final yields of 200 to 500 pk.f.u. per cell; at 40° there was no growth of infectious virus. In contrast, vaccinia virus grew equally well at either temperature. The temperature-sensitive phase of the growth cycle was delineated by experiments involving temperature-shift. The first sensitive event was at 4 hr after infection, the time at which virus DNA synthesis normally begins at 35°, and the last was within 1 hr of the onset of virus maturation. Other features of the growth cycle at 40° were the absence of virus DNA production, the delayed appearance of the LS-antigen complex and the haemagglutinin, and the absence of any evidence of particle formation. Analysis of the temperature-shift experiments suggested that production of the LS-antigen complex and particle development were both directly involved in the late event.


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