FIG 2.

Effects of the HA ssNCRs on influenza A virus replication efficiency. (A) Schematic representation of recombinant WSN HA vRNAs with subtype-specific NCR substitutions at both the 3′ and 5′ ends. The green and blue boxes represent the 12 and 13 terminal promoter nucleotides at the 3′ and 5′ ends, respectively. The red boxes represent ORFs of WSN virus HA. The nucleotides shown between the promoter nucleotides and the ORFs are the subtype-specific NCRs of the indicated HA subtypes. (B) Growth curves of rescued viruses in MDCK cells. WT and mutant WSN viruses were plaque purified and passaged for two generations. MDCK cells were infected with virus at an MOI of 0.001. At 0 h, 12 h, 24 h, 48 h, 60 h, and 72 h after infection, the virus titer in the supernatant was determined by plaque assay of the MDCK cells. The error bars indicate the standard deviations of three independent experiments. H4-NCR, H6-NCR, and H7-NCR were not able to be rescued. (C) Virus titers were determined at 12 h p.i. **, P < 0.01; two-tailed Student's t test.