Mutants defective in herpes simplex virus type 2 ICP4: isolation and preliminary characterization.

Vero cells were biochemically transformed with the gene encoding ICP4 of herpes simplex virus type 2 (HSV-2). These cells were used as permissive hosts to isolate and propagate HSV-2 mutants defective in this gene. Two mutants, designated hr259 and hr79, were isolated. Neither mutant grew in nontransformed Vero cells, but both grew to near wild-type levels in HSV-2 ICP4-expressing cells. hr259 contains a deletion of about 0.6 kilobases which eliminates the mRNA start site of the ICP4 gene. hr79 contains a mutation which maps by marker rescue to the portion of the ICP4 gene encoding the carboxy-terminal half of the protein. Although hr259 failed to generate any detectable ICP4 mRNA in nontransformed Vero cells, hr79 encoded an ICP4 mRNA which is wild type with respect to size. In nontransformed Vero cells infected with hr259, only ICP0, ICP6, ICP22, and ICP27 were readily detectable. In the case of hr79, a truncated form of ICP4 appeared to be made in addition to ICP0, ICP6, ICP22, and ICP27. Both hr259 and hr79 grew efficiently on cell lines transformed with the ICP4 gene of HSV-1 as evidenced by plating efficiencies and single-burst experiments. Similarly, cells transformed with the ICP4 gene of HSV-2 served as efficient hosts for the growth of d120, HSV-1 ICP4 deletion mutant.