Genetic Variation During Persistent Reovirus Infection: Presence of Extragenically Suppressed Temperature-Sensitive Lesions in Wild-Type Virus Isolated from Persistently Infected L Cells

Abstract

Persistent reovirus infection of L cells was established with a serially passaged stock of temperature-sensitive ( ts ) mutant C(447) containing greater than 90% defective interfering particles. Within a month after establishment of the carrier culture, the ts mutant was replaced by virus that expressed the wild-type ( ts + ) temperature phenotype (R. Ahmed and A. F. Graham, J. Virol. 23 :250-262, 1977). To determine whether the ts + phenotype of the virus was due to intragenic reversion or to the presence of an extragenic mutation suppressing the original ts defect, several clones were backcrossed to wild-type reovirus, and the progeny of each cross were screened for temperature sensitivity. The results indicated that the original ts C lesion had reverted. However, in two of the seven clones examined, new ts lesions were found. These new ts lesions appeared phenotypically as ts + due to the presence of extragenic suppressor mutations. Temperature-sensitive mutants representing three different groups were rescued from one suppressed clone, indicating that this ts + clone contained multiple ts lesions. Among the ts mutants rescued were the initial isolates of a new recombination group which we have designated H. Some of the ts mutants rescued from the suppressed clones are capable of interfering with the growth of wild-type reovirus and may play a role in maintaining the carrier state. The results of this study show that persistently infected L cells contain a genetically heterogeneous population of reovirus even though all virus clones express the ts + phenotype. It is thus critical to distinguish between genotype and phenotype when analyzing viruses that emerge during persistent infection.