Characterization of a foot-and-mouth disease mutant temperature-sensitive for viral RNA synthesis

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Abstract
A temperature-sensitive (ts) mutant of foot-and-mouth disease virus (FMDV) did not produce RNA polymerase activity nor synthesize viral RNA when incubated in cells solely at the nonpermissive temperature (38.5° C). Infected cells initially incubated at 38.5° C and then shifted down to 33° C synthesized increased amounts of viral RNA at earlier times compared to infected cells kept at 33° C throughout, indicating that RNA polymerase precursors were synthesized at 38.5° C. In cells shifted up to 38.5° C from 33° C, the total amount of viral RNA synthesized after infection increased sharply for about 15 minutes and then rapidly decreased over the next 2 hours. RNA polymerase activity presented a similar pattern in its initial twofold increase and subsequent rapid decrease. Pulse labeling experiments showed that mutant viral RNA synthesis continued at a diminishing rate for 2 hours in cells shifted up to 38.5° C. The data from temperature shift experiments indicated that essentially only the mutant RNA formed after shift-up was degraded. The FMDVts mutant is apparently additionally defective in being unable to protect viral RNA synthesized after shift-up to 38.5° C.