Primary structure of the polymerase acidic (PA) gene of an influenza B virus (B/Sing/222/79).

A DNA copy of influenza B/Singapore/222/79 viral RNA segment 3 containing the gene coding for the polymerase acidic (PA) protein has been cloned in Escherichia coli plasmid pBR322, and its nucleotide sequence has been determined. The cDNA clone was incomplete and contained 1810 nucleotides (nt 396 to 2205). The remaining nucleotide sequence at both 5' and 3' ends of B PA gene was obtained by sequencing the viral RNA (minus sense) and messenger RNA (plus sense) using oligonucleotide primers. The influenza B PA gene contains 2304 nucleotides and codes for a protein of 725 amino acids with a molecular weight of 83,000. The predicted influenza B PA protein is less acidic than the influenza A PA protein. Computer alignment of the influenza B PA amino acid sequence with that of influenza A PA (A/PR/8/34) revealed an overall 38% direct homology which increases to 45% in the carboxyl terminus half of the protein. In addition, comparison of the secondary structural elements, hydropathy profile, and isofunctional amino acid changes between B PA and A PA proteins demonstrated a strong structural and possibly functional conservation between these two proteins. These data suggest that PA genes of influenza A and B viruses arose from a common ancestor gene.