Isolation and characterization of naturally occurring deletion mutants of moloney murine sarcoma virus

Abstract

Moloney murine sarcoma virus strain 124 (MSV-124) possesses the ability to transform cells but requires a type-C RNA virus as a helper virus for its propagation as an infectious virus. We inquired whether natural selection during virus replication and transmission would lead to alteration of MSV-124 genetic information. Individual viruses were cloned in non-roducer transformants from an MSV-124 stock that had undergone a single or multiple cycles of infection. Analysis of the ability of individual MSV-124 transformants to express helper viral gag gene products indicated the selection of variants that had lost the capacity to express some or all such MSV-124 coded information. The genomic size of individual MSV-124 variants was determined by electrophoretic and hybridization analysis of unintegrated proviral DNA. Viruses isolated following multiple cycles of infection were shown to contain deletions of up to 35% of the parental MSV-124 genome. Moreover, there was a striking natural selection for such mutants. Thus, the evidence indicates that MSV-124 deletion mutants possessed a significant growth advantage relative to the parental MSV-124 genome. High molecular weight DNA of nonproducer transformants isolated following infection at low multiplicity (<0.01) with parental MSV-124, often contained more than one copy of the viral genome. Possible interpretations of these findings are discussed.