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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 ofthe 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 Ml 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). We report here the results of experiments designed to determine the precise 5'-terminal nucleotides of the M2 mRNA, to demonstrate directly that translation of the M2 mRNA could occur in the second open frame, and to investigate whether the M2 mRNA was analogous to the NS2 mRNA in having an interrupted region.

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