FIG. 4.

Sialidase activity of mutant viruses. Sialidase activity was determined with the 2′-(4-methylumbelliferyl)-α-d-N-acetylneuraminic acid substrate as described in Materials and Methods. Specific activity was calculated by normalizing NA activity to the level of virus particles, as determined by immunoblotting viral proteins (separated by SDS-PAGE) with antibody against M1, followed by densitometric analysis. Relative NA activity, taking that of WT virus as 100%, was calculated from the results from three independent experiments, with an average variation of less than 10%.