A mutation in the PA protein gene of cold-adapted B/Ann Arbor/1/66 influenza virus associated with reversion of temperature sensitivity and attenuated virulence.

Reassortant SG3 inherits only the acidic polymerase (PA) protein gene from the cold-adapted B/AA/1/66 influenza virus (ca B/AA/1/66) and all remaining genes from a virulent, wild-type virus. This reassortant demonstrates attenuated virulence in ferrets and expresses a ts phenotype characteristic of the ca parent. During virulence evaluation of SG3, a virulent, non-ts revertant virus (designated SG3rFL) was isolated from the lungs of one ferret. In order to determine whether the reversion of SG3 resulted from mutation of the PA gene and/or as the result of extragenic supressor mutations, the revertant PA gene of SG3rFL was transferred to a reassortant (SG3r) inheriting only the revertant PA gene from SG3rFL and all remaining genes from SG3. Reassortant SG3r was non-ts and virulent, indicating that mutation of the PA gene was sufficient for the reversion of the ts and attenuation phenotypes expressed by SG3rFL. The nucleotide and predicted amino acid sequences of the SG3rFL PA gene were determined and compared to those of wt and ca B/AA/1/66. The predicted PA proteins of wt and ca B/AA/1/66 are known to differ by six amino acid substitutions including a valine to methionine substitution at residue 431. The PA proteins of ca B/AA/1/66 and SG3rFL were distinguished by only the single amino acid substitution of methionine to isoluecine also occurring at residue 431. Thus, the methionine residue was implicated in the attenuation of ca B/AA/1/66 and its reassortants. The hydropathic properties of valine, isoleucine, and methionine suggested that reversion involved the restoration of hydrophobic character at this site.