Enhanced polymerase activity of SC35M.(A and B) Polymerase activity of ribonucleoprotein complexes reconstituted from PB1, PB2, PA, and NP plasmids of SC35, SC35M, and their combinations (A) and of their single-point mutants (B). (C) Correlation of polymerase activity of recombinant virus with virulence. The mouse LD50 of viruses was plotted (inverted axis) against their polymerase activities. The polymerase activities were determined by luciferase reporter assay in four independent experiments.