HVJ wild-type virus, in which the F protein is activated by trypsin but not by elastase, was spontaneously converted to a mutant with an F protein characterized by being activated by elastase alone. This spontaneous mutation generally occurred during serial passages of cells persistently infected with HVJ, even though the cells were first established by infection with plaque-purified wild-type virus. Multiple-cycle replication, plaque formation, haemolysis and SDS-polyacrylamide gel electrophoretic (SDS-PAGE) analysis showed that all the elastase-activated mutants isolated from HVJ carrier cells no longer required trypsin for F protein activation. At early passages, these protease activation mutants did not show temperature-sensitive () growth, while at a later stage the mutants, together with the mutation, appeared dominant. The frequency of such a protease activation mutation during passage in the HVJ carrier cells seemed to depend on the cell species, but was increased when compared to lytic infections.


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