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Temperature-Sensitive Mutants Isolated from L Cells Persistently Infected with Newcastle Disease Virus

Virus mutants (NDVpi) isolated from L cells persistently infected with the Herts strain of Newcastle disease virus have been previously reported by this laboratory to differ from the wild-type virus (NDVo) in several physical and biological properties. It has now been determined that, in addition to these differences, the NDVpi mutants are also spontaneously selected temperature-sensitive mutants. The temperature sensitivity of 10 NDVpi clones was confirmed by temperature inhibition, plaquing efficiency, and single-cycle yield experiments. The cut-off temperature, at which more than 90% of virus replication is inhibited was between 41 and 42 C. All 10 NDVpi clones were also found to be defective in virus-specific ribonucleic acid (RNA) synthesis in infected chick embryo cells at 42 C and are tentatively classified as RNA−. The possible relationships of the temperature sensitivity, the other NDVpi properties, and the maintenance of the persistently infected state are discussed.