Localization of foot-and-mouth disease--RNA synthesis on newly formed cellular smooth membranous vacuoles.

Viral RNA synthesis in foot-and-mouth disease infected bovine kidney cell cultures was associated throughout the infectious period with newly formed smooth membranous vacuoles. Membrane formation was measured by choline uptake. The site of RNA synthesis was determined by electron microscopic examination of autoradiograms of incorporated [3H] uridine. Both membrane formation and RNA synthesis became significant at 2.5 hours postinfection, but membrane formation increased steadily to 4.5 hours while RNA synthesis peaked at 3.5 hours. Percent density distributions of developed silver grains on autoradiograms showed that almost all RNA synthesis was concentrated on the smooth vacuoles of infected cells. Histogram analysis of grain density distributions established that the sites of RNA synthesis was the vacuolar membrane. The newly formed smooth membrane-bound vacuoles were not seen to coalesce into the large vacuolated areas typical of poliovirus cytopathogenicity.