Fig. 1

Evolution of HA RBS and the mutability of residue 190. a The frequencies of several representative substitutions in HA RBS in naturally circulating human H3N2 strains are shown. b Residues that are involved in selective sweeps during the natural evolution of human H3N2 viruses were analyzed for the entire HA and for the RBS, and were further classified into “with reversion”, “multiple sweep”, and “single sweep” (see Methods). The cartoon arrows on the right demonstrate several scenarios for the classification scheme. The evolutionary timeline of a given residue is shown from the left to right, where different colors represent different dominant amino-acid variants at the given residue. “R” indicates the occurrence of reversion. c Effect of D190E on viral replication fitness for different chimeric H3N2 strains was examined by virus rescue experiment. Virus titer was measured by TCID₅₀. Error bars indicate the standard deviation of three independent experiments. The green dashed line represents the lower detection limit