Selection and characterization of a neuraminidase-minus mutant of influenza virus and its rescue by cloned neuraminidase genes.

A neuraminidase (NA)-deficient mutant, designated NWS-Mvi, of the reassortant influenza virus A/NWS/33HA-A/tern/Australia/G70c/75NA (H1N9), was selected by passaging virus in MDCK cells in a medium containing neuraminidase from the bacterium Micromonospora viridifaciens and polyclonal antiserum against the influenza NA. Growth of the resulting mutant virus is dependent on the addition of neuraminidase to the medium. Western blot analysis showed that the neuraminidase protein was absent from the mutant virus particles, and Northern hybridization showed that RNA segment 6, which contains the coding information for the NA, had undergone massive deletion. Viral protein synthesis in cells infected with the mutant virus was not dependent on the addition of neuraminidase. In the absence of a functional NA, the NWS-Mvi mutant virus can infect MDCK cells with normal cytopathic effects. This neuraminidase-minus influenza virus serves as an excellent source of parent virus for reverse genetics experiments involving genes that encode a functional neuraminidase.