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

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 degrees C). Infected cells initially incubated at 38.5 degrees C and then shifted down to 33 degrees C synthesized increased amounts of viral RNA at earlier times compared to infected cells kept at 33 degrees C throughout, indicating that RNA polymerase precursors were synthesized at 38.5 degrees C. In cells shifted up to 38.5 degrees C from 33 degrees C, the total amount of viral RNA synthesized after infection increased sharply for about 15 minutes and then rapidly increased 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 degrees C. The data from temperature after shift-up was degraded. The FMDV ts mutant is apparently additionally defective in being unable to protect viral RNA synthesized after shift-up to 38.5 degrees C.