Deduced amino acid sequences at the haemagglutinin cleavage site of avian influenza A viruses of H5 and H7 subtypes.

The amino acid sequences at the haemagglutinin cleavage sites of 9 avian influenza A viruses of H5 subtype (5 high and 4 low pathogenicity for chickens) and 21 of H7 subtype (13 high and 8 low pathogenicity for chickens) were determined by direct RNA sequencing, PCR amplification sequencing or both. None of the viruses of low pathogenicity had multiple basic amino acids at the cleavage site. All highly pathogenic viruses had an insert of basic amino acids at the cleavage site, except A/chicken/Scotland/59 (H5N1) for which the multiple basic amino acids appeared as substitutions and not insertions. All highly pathogenic viruses examined conformed to the amino acid motif of R-X-R/K-R at the cleavage site which is considered to be essential for high pathogenicity in chickens, with the notable exception of highly pathogenic virus A/turkey/England/50-92/91 (H5N1) which had the sequence R-K-R-K-T-R adjacent to the cleavage site.