Thermosensitivity of frog virus 3 genome expression: defect in early transcription.

The influence of temperature on the transcription of the frog virus 3 genome was studied in CHO cells infected both at 29 and at 37 degrees, the nonpermissive temperature for virus multiplication. It was definitely established that late genes were not transcribed at 37 degrees. Although immediate early genes were expressed at 37 degrees, their transcription was altered but there was no sequestration of mRNAs in the nucleus which could impair their translation; these viral mRNAs were also efficiently translated in vitro. These results indicate that an immediate early viral protein involved in the transcription of delayed early genes is likely to be thermosensitive. Furthermore, one event taking place at the very beginning of the infection, possibly related to the activity of a viral structural component, facilitates the transcription of immediate early genes at 29 degrees and this step is partially impaired at 37 degrees.