Temperature-sensitive mutants of fowl plague virus (influenza A) generated by undiluted passages at 33 degrees C.

Temperature-sensitive (ts) mutants obtained by undiluted passages of fowl plague virus at 33 degrees C have their defects located mainly in RNA segments 3, 4 and 8 as determined by rescue to wild-type with standard ts mutants. This result is different from that obtained after treatment of virus with mutagens, where the frequency of mutations follows roughly the target size of the RNA segments. Many isolates generated after undiluted passages at 33 degrees C, which seem to have mutations in RNA segments 3 and 4, can be rescued to wild-type. This occurs, however, with certain defined standard ts mutants having a defect in RNA segment 4, but not by other segment 4 mutants. One such mutant, ts 1/93 (ts defect in segment 3), interferes with the multiplication of ts 227 (ts defect in segment 4) at the permissive temperature, presumably at the level of vRNA synthesis, preventing reassortment to wild-type. Similarly, ts 263 (ts defect in segment 3) interferes with the multiplication of ts 1/1 (ts defect in segment 4). For other such interfering mutants, the mechanism preventing reassortment to wild-type is different from that of ts 1/93 or ts 1/1, but is not yet understood. Thus, the number of mutations as determined by rescue with standard ts mutants in isolates obtained by undiluted passages is overestimated due to intrinsic interference.