There is a significant difference in the ability of human influenza A virus H1N1 strains isolated up to 1977 and those isolated later to rescue temperature-sensitive mutants of fowl plague virus with a defect in the nucleoprotein (NP) gene. Therefore the NP genes of five human H1N1 and H3N2 influenza A virus strains, isolated between 1950 and 1978, have been sequenced. By comparison with previous and more recent isolates, and evolutionary pathway has been established. Three amino acid replacements were found which might be responsible for the functional difference between the USSR (1977) and the Brazil (1978) strains. The California (H1N1) strain isolated in 1978 had acquired by reassortment the NP gene of a human H3N2 virus circulating at about 1977 as had been previously suggested by investigations involving RNase fingerprint or hybridization techniques.


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