Evidence for intracistronic complementation of the product of the influenza virus gene Ptra (P3 of fowl plague virus).

The ts lesion of the fowl plague virus (FPV) mutants ts 18 and ts 236 has been located in RNA segment 2 (Ptra gene, corresponding to P3). After double-infection with these mutants and ts 90 or ts 93, which also carry a ts lesion in segment 2, plaques were formed at the non-permissive temperature (40 degrees C). These plaques cannot be passaged at 40 degrees C and exhibit a morphology which differs from those formed by the wild-type virus. The yield of infectious particles after double-infection shows a non-linear correlation between the plaque number and dilution, indicating that at least two particles are needed for infection of a cell. All experimental evidence points to an intracistronic complementation within the P3 protein.