An attenuated mutant of Venezuelan encephalitis virus: Biochemical alterations and their genetic association with attenuation

A temperature-sensitive mutant of Venezuelan encephalitis virus derived by chemical mutagenesis from the hamster-virulent 68U201 wild-type (wt) strain was previously found to be attenuated. This mutant, is 126, replicated in infected hamsters and elicited production of protective antibodies. Further phenotypic differences between is 126 and 68U201 wt have been characterized in an attempt to localize the genetic basis of the mutant's attenuated virulence. The mutant was shown to differ from the parent virus with respect to virion surface structure-dependent characteristics: temperature lability, plaque sizes in Vero cells, and binding properties to hydroxylapatite. The surface difference was identified by isoelectric focusing of the virion envelope glycoproteins as an alteration in the E₁ glycoprotein. The common genetic basis of all these phenotypic differences was demonstrated by the isolation of independently arising, stable genetic revertants of ts 126, which exhibited characteristics identical in every respect to 68U201 wt. It appears from these studies that the mutation which gave rise to the is 126 mutant virus occurred in the structural gene coding for the E₁ envelope glycoprotein and that the resultant phenotypic alteration in this glycoprotein is genetically associated with the mutant's lack of virulence.