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The complete nucleotide sequence has been obtained of the H1 hemagglutinin (HA) gene of a high-yielding (H) mutant of the A/NJ/11/76(H1N1) strain of swine influenza virus in studies of a viral reassortant (X-53a) bearing this gene. This determination has permitted comparison with human influenza H1N1 prototype viruses A/WSN/33 and A/PR/8/34, with which 80% and 94% amino acid homology was found between HA1 and HA2, respectively. Partial sequences have been determined for other viral reassortants containing either H or L (low-yielding phenotype) genes derived from A/NJ/11/76. Sequence of the HA1 region of an L mutant prototype was virtually completed and differed from that of the H mutant by only four amino acid changes. Sequence analysis of four other viruses was restricted to regions of the HA with which monoclonal antibodies capable of distinguishing L and H mutants are presumed to react. Therefore, changes in these sequences are relevant to changes in viral phenotype. Change at residue 155 from Gly to Glu is associated with change from L to H HA phenotype. This site, structurally equivalent to amino acid 158 on the Wiley et al. HA model [Wiley, D. C., Wilson, I. A. &amp; Skehel, J. J. (1981) Nature (London) 289, 373-378] is near the tip of the HA monomer adjacent to the proposed receptor binding site and therefore credibly could influence both viral antigenicity and replication. Because both L and H variants exist in nature and because revertants may be selected in the laboratory as replication variants in the absence of immunoselection, these studies provide evidence for fortuitous antigenic change in association with change in biological function, which is determined by a single base change.

Hemagglutinin of swine influenza virus: a single amino acid change pleiotropically affects viral antigenicity and replication.