Sequences of mRNAs derived from genome RNA segment 7 of influenza virus: Colinear and interrupted mRNAs code for overlapping proteins

RNA segment 7 of the influenza A virus genome codes for at least two proteins, M₁ and M₂, which are synthesized from separate mRNA species. Sequence analysis of the M₂ mRNA has shown that it contains an interrupted sequence of 689 nucleotides. The ~ 51 virus-specific nucleotides comprising the 5'-end leader sequence of the M₂ mRNA are the same as those found at the 5' end of the colinear M₁ mRNA. Following the leader sequence of the M₂ mRNA, where is a 271 nucleotide body region that is 3' coterminal with the M₁ mRNA. Another small potential mRNA (mRNA₃) related to RNA segment 7 has been found. mRNA₃ has a leader sequence of ~ 11 virus-specific nucleotides that are the same as the 5' end of the M₁ and M₂ mRNAs, followed by an interrupted sequence of 729 nucleotides, and then a body region of ~ 271 nucleotides that is the same as that of the M₂ mRNA. The nucleotide sequences found at the junctions of the interrupted sequences in M₂ mRNA and mRNA₃ are similar to those found at the splicing points of intervening sequences in eukaryotic mRNAs. In addition, both mRNAs contain 10-15 heterogeneous nonviral nucleotides at their 5' ends that appear to be derived from cellular RNAs used for priming the transcription of viral RNAs. Because the 5'-end sequences of the M₁ mRNA and the M₂ mRNA are the same and share the 5' proximal initiation codon for protein synthesis, the first nine amino acids would be the same in the M₁ and M₂ protein and then the sequences would diverge. The ~ 271-nucleotide body region of the M₂ mRNA can be translated in the + 1 reading frame, and the sequence indicates that M₁ and M₂ overlap by 14 amino acids. The coding potential of the mRNA₃ is for only nine amino acids, and these would be identical to the COOH-terminal region of the membrane protein (M₁).