FIG. 3.

Sialidase activity of the parental AM2AL3 and K4 viruses and the AL3(MaKS)-13 and K4(MaKS)-13 mutants. For each sample, virus (5 × 10² PFU) was incubated in duplicate for 1 h at 37°C in the presence of a fluorogenic sialidase substrate (4-methylumbelliferyl-a-d-N-acetylneuraminic acid). The fluorescence of released 4-methylumbelliferone was determined with a fluorometer (Labsystems Fluoroskan II) with excitation at 360 nm and emission at 460 nm.