Balanced hemagglutinin and neuraminidase activities are critical for efficient replication of influenza A virus

The SD0 mutant of influenza virus A/WSN/33 (WSN), characterized by a 24-ami no-acid deletion in the neuraminidase (NA) stalk, does not grow in embryona ted chicken eggs because of defective NA function. Continuous passage of SD O in eggs yielded 10 independent clones that replicated efficiently. Charac terization of these egg-adapted viruses showed that five of the viruses con tained insertions in the NA gene from the PB1, PB2, or NP gene, in the regi on Linking the transmembrane and catalytic head domains, demonstrating that recombination of influenza viral RNA segments occurs relatively frequently . The other five viruses did not contain insertions in this region but disp layed decreased binding affinity toward sialylglycoconjugates, compared wit h the binding properties of the parental virus. Sequence analysis of one of the latter viruses revealed mutations in the hemagglutinin (HA) gene, at s ites in close proximity to the sialic acid receptor-binding pocket. These m utations appear to compensate for reduced NA function due to stalk deletion s. Thus, balanced HA-NA functions are necessary for efficient influenza vir us replication.