Two chemically induced mutants with an alteration in the susceptibility of glycoprotein F to proteolytic cleavage have been isolated from the apathogenic strains of La Sota and Ulster of Newcastle disease virus. In contrast to the La Sota wild type, cleavage of the precursor F and activation of cell fusing activity and infectivity take place if the mutant is grown in MDBK and BHK21-F cells. The mutant is, therefore, able to undergo multiple replication cycles in cells nonpermissive for the wild type. This increase in host range is paralleled by an increase in pathogenicity for chick embryos. The increase in host range of the Ulster mutant is less distinct. This mutant, which does not differ in pathogenicity from its wild type, produces in MDBK cells incompletely activated virus containing predominantly glycoprotein HN in the uncleaved and glycoprotein F in the cleaved form. The data support the concept that the susceptibility of the virus glycoproteins to proteolytic activation is an important factor in determining the pathogenicity of this virus.


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