THERMOLYSIN ACTIVATION MUTANTS WITH CHANGES IN THE FUSOGENIC REGION OF AN INFLUENZA-VIRUS HEMAGGLUTININ

Influenza virus A/seal/Mass/1/80 (H7N7) mutants were obtained; the hem agglutinins (HAs) of the mutants were not activated by trypsin, as in the wild-type virus, but by thermolysin. The mutants grew efficiently under multiple replication cycle conditions and formed plaques in chic ken embryo cells only when thermolysin was added to the culture medium . They exhibited hemolytic activity and induced protective immunity in chickens after an asymptomatic course of infection. Nucleotide sequen cing of the HA gene and direct amino acid sequencing showed that inser tion of a single leucine into the fusion peptide of the HA2 chain clos e to the cleavage site and a shift of the cleavage site toward the C t erminus by one amino acid were responsible for the changes in the biol ogical properties of the thermolysin activation mutants. Revertants co uld be obtained when trypsin or trypsin-like endoproteases were presen t in the virus-producing system.