Characterization of a cold-sensitive (cs) recombinant between two influenza a strains

Recombinants between fowl plague virus (FPV, H7N1) and the Hong Kong (H3N2) or Singapore (H2N2) influenza virus strains carrying the hemagglutinin of FPV and the neuraminidase of the human strains form only very tiny plaques at 33°, but normal plaques at 37°. One recombinant (113/Ho) has been studied in more detail. It multiplies only very slowly at 33°, the nonpermissive temperature. Adsorption and penetration are normal at 33°, but synthesis of protein is impeded. Temperature-shift experiments suggest that the synthesis of viral mRNA is slowed at 33°. 113/Ho does not agglutinate chicken erythrocytes at 40°, as the parent viruses do. 113/Ho can be adapted to grow normally at 33°. The frequency of adaptation is comparable to reversion of a single point mutation (ca. 10(−5)). Recombinants which grow well at 37° but not at 33° are called cold-sensitive (cs) recombinants.