Sequential mutations in the NS genes of influenza virus field strains

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Abstract
The complete nucleotide sequences of the NS genes from 3 human influenza viruses, A/FM/1/47 (H1N1), A/FW/1/50 (H1N1) and A/USSR/90/77 (H1N1), were determined. Only 5 single-base differences were found within the sequences of the A/FW/1/50 and A/USSR/90/77 NS genes, thus confirming earlier data suggesting that the 1977 H1N1 viruses are closely related to virus strains that were circulating around 1950. Comparison of all 3 sequences with those from A/PR/8/34 and A/Udorn/72 viruses illustrates that these genes (with the exception of that of the A/USSR/90/77 strain) evolve through cumulative base changes along a singe common lineage. A nucleotide sequence variation of .apprx. 2.2-3.4%/10 yr was determined for the NS gene segments. Extensive size variation was also observed among the NS1 proteins of the various human viruses. The A/FM/1/47 NS1 protein, which consists of 202 amino acids, is 15% shorter than the A/Udorn/72 NS1 protein, which consists of 237 amino acids.