Influenza B virus genome: Sequences and structural organization of RNA segment 8 and the mRNAs coding for the NS₁ and NS₂ proteins

Double-stranded DNA derived from influenza B virus genome RNA segment 8, which codes for the NS₁ and NS₂ proteins, was constructed by hybridization of full-length cDNA copies of RNA segment 8 and of the NS₁ mRNA. This DNA was cloned in plasmid pBR322 and sequenced. The NS₁ mRNA (~1,080 viral nucleotides) contains nonviral nucleotides at its 5' end and is capable of coding for a protein of 281 amino acids. Sequencing of the NS₂ mRNA has shown that it contains an interrupted sequence of 655 nucleotides and is most likely synthesized by a splicing mechanism. The first ~75 virus-specific nucleotides at the 5' end of the NS₂ mRNA are the same as are found at the 5'-end of the NS₁ mRNA. This region contains the initiation codon for protein synthesis and coding information for 10 amino acids common to the two proteins. The ~350-nucleotide body region of the NS₂ mRNA can be translated in the +1 reading frame, and the sequence indicates that the NS₁ and NS₂ protein-coding regions overlap by 52 amino acids translated from different reading frames. Thus, between the influenza A and B viruses, the organization of the NS₁ and NS₂ mRNA and the sizes of the NS₂ mRNA and protein are conserved despite the larger size of the influenza B virus RNA segment, NS₁ mRNA, and NS₁ protein.