IDENTIFICATION OF MUTATIONS CONTRIBUTING TO THE REDUCED VIRULENCE OF A MODIFIED STRAIN OF RESPIRATORY SYNCYTIAL VIRUS

The nucleotide sequences of the genome of the RSS-2 wild type strain o f respiratory syncytial (RS) virus, which is known to induce upper res piratory tract infection in adults, and that of the attenuated ts1C ca ndidate vaccine derived from if by three cycles of mutagenesis and sel ection of temperature-sensitive (ts) mutants, have been determined. Co mparison of the sequences has located the genetic changes which contri bute to the reduced pathogenicity in adults of the candidate vaccine. Thirty-seven nucleotide changes distinguish the wild type and ts1C, 13 of which confer amino acid substitutions; no mutations are present in extragenic regions. Partial nucleotide sequencing of the genomes of t he first stage ts mutant (ts1A) and the second stage ts mutant (ts1B), which were intermediates in the derivation of the third stage mutant ts1C, established that five mutations resulting in amino acid substitu tions had been induced in the first cycle of mutagenesis, one in the s econd cycle, and seven in the third cycle. The unique mutation differe ntiating ts1B from ts1A substitutes an alanine for a threonine at resi due 736 in the polymerase (L) protein. The occurrence of a mutation in ts1C inducing substitution of a phenylalanine for a serine residue at an adjacent site (731) suggests that mutations in this region of the polymerase can have significant attenuating effects. The data suggest also that a mutation in the F gene may contribute to the attenuated ph enotype. Copyright (C) 1996 Elsevier Science Ltd.