Figure 1.

Complete coding sequence of the HA gene of the 1918 influenza virus. The sequence for A/South Carolina/1/18 is shown with a theoretical translation of the HA1 and HA2 domains. The numbering of the nucleotide sequence is aligned to PR/8/34 and refers to the sequence of the gene in the sense (mRNA) orientation. The sequence coding for the signal peptide is underlined. The cleavage site (nucleotides 1,062–1,064) between the HA1 and HA2 domains is shown in bold. Sequence differences between this strain and the other 1918 strains are shown as double-underlined nucleotides (nucleotides 416 and 748). The sequences were confirmed by sequencing overlapping RT-PCR products and by replicate RT-PCRs for each case. The GenBank accession number is AF117241 for A/South Carolina/1/18, AF116576 for A/New York/1/18, and AF116575 for A/Brevig Mission/1/18. The theoretical translation of the gene is shown above the nucleotide sequence. Boxed amino acids indicate potential glycosylation sites as predicted by the sequence (26). Receptor-binding sites (open diamonds; ref. 23), C_b antigenic site (open circles), S_a antigenic site (closed squares), S_b antigenic site (closed diamonds), and C_a antigenic site (closed triangles; refs. 28 and 37). Some of the receptor-binding residues are also in known antigenic sites. For these sites, symbols for both are shown.