Evolution of influenza A and B viruses: conservation of structural features in the hemagglutinin genes.

The complete nucleotide sequence of the hemagglutinin (HA) gene of a type B influenza virus (B/Lee/40) was obtained by using cloned cDNA derived from the RNA segment. The gene is 1,882 nucleotides long and can code for a protein precursor of 584 amino acids. Structural features common to type A virus HAs are also conserved in the B virus HA. These include a hydrophobic signal peptide, hydrophobic NH2 and COOH termini of the HA2 subunit, and a HA1/HA2 cleavage site involving an arginine residue. The sequence of the B HA gene and its deduced amino acid sequence were compared to those of a type A influenza virus (A/PR/8/34). When these two genes were aligned, it was found that 24% of the amino acids in the HA1 subunits and 39% of the amino acids in the HA2 subunits are conserved. This degree of relatedness between type B virus and type A virus HAs (intertypic comparison) is similar to the homologies observed among certain type A virus HAs (intratypic comparison). A close evolutionary relationship is therefore suggested between the HAs of type A and type B influenza viruses.