Oligo(A) sequences of human respiratory syncytial virus G protein gene: assessment of their genetic stability in frameshift mutants.

We have described previously antibody-resistant mutants of the human respiratory syncytial virus Long strain that contained frameshift changes generated by deletions or insertions of a single adenosine in oligo(A) tracts (mRNA sense) of the G protein gene. Since these mutations introduced drastic structural and antigenic changes in the G protein C-terminal third, we decided to test the mutant stability by passaging the viruses in either the presence or the absence of selective antibody. Two such mutants (R63/1/2/3 and R63/2/4/8), with a single reading frame shift, reverted after a few passages in the absence of antibody to the wild-type genotype, by insertion of an A at the same homopolymeric tract as in the original deletion. In contrast, a double frameshift mutant (R63/2/4/1), generated by deletion of an A after nucleotide 623 and insertion of another A seven triplets later, was stably maintained after passage in either the absence or the presence of antibody. The stability of this mutant was manifested in its capacity to gradually displace the Long strain from mixed infections and by the fact that mutant R63/2/4/8 acquired the genotype of R63/2/4/1 after several passages in the presence of antibody. These results were indicative of genetic instability in the oligo(A) tract length of certain G protein mutants, which resulted in frameshift changes. The frequency of such errors among the viral RNA population obtained from a single infectious cycle was estimated to be lower than 1%. The relevance of these results for respiratory syncytial virus evolution is discussed.