The complete nucleotide sequence of a cloned full-length DNA copy of genome RNA segment 7 of influenza B/Lee/40 virus has been determined. The messenger RNA (∼ 1175 viral nucleotides) for the nonglycosylated membrane protein of the virus (M1) is capable of coding for a protein of 248 amino acids, of which 63 are conserved between influenza A and B viruses. An internal hydrophobic domain, which includes three cysteine residues in the same positions, is also conserved between the M1 proteins of the two viruses and may be of functional significance. A second coding region overlaps that of the influenza B M1 protein by 86 amino acids. This region could code for a maximum of 195 amino acids in the +2 reading frame, depending on whether or not the mRNA is spliced and the location of the initiating methionine residue. This region is likely to code for a protein analogous to the influenza A virus M2 protein. No obvious homology has been detected in the deduced amino sequences of the M2 proteins of the two viruses. The size of the M1 coding regions is conserved between influenza A and B viruses, however the B virus segment 7 is larger, with the difference largely due to a longer M2 coding region.
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