Sequence of RNA segment 7 of the influenza B virus genome: Partial amino acid homology between the membrane proteins (M₁) of Influenza A and B viruses and conservation of a second open reading frame

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.