The complete sequence of a full-length cloned DNA copy of the influenza virus A/Udorn/72 (H3N2) RNA segment 7 has been determined. A second open reading frame has been found which overlaps the membrane protein (M1) sequence by 68 nucleotides. This second reading frame could code, in the +1 reading frame, for a protein (M2) with a maximum of 97 amino acids depending on whether there is splicing of the mRNA and the methionine residue used for initiation of protein synthesis. Comparison of the present H3N2 sequence with the previously published sequence (G. Winter and S. Fields, Nucleic Acids Res., 8, 1965-1975, 1980; H. Allen, J. McCauley, M. Waterfield, and M.-J. Gething, Virology, 107, 548-551, 1980) of RNA segment 7 of A/PR/8/34 (H1N1), a strain isolated 38 years earlier, has shown that the amino acid sequence for the M1 protein has been greatly conserved. In a total of 252 amino acids, only seven amino acid changes have occurred, of which only one results in a change in charge. In the amino acid sequences coded by the second open reading frame, changes were more common between strains, 11 out of a total of 96 amino acids.
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