Construction and characterization of a herpes simplex virus type 1 mutant unable to transinduce immediate-early gene expression

A herpes simplex virus mutant, in1814, possessing a 12-base-pair insertion in the gene encoding the transinducing factor Vmw65 has been constructed. The insertion abolished the ability of Vmw65 to transinduce immediate-early (IE) gene expression and to form a protein-DNA complex with cell proteins and the IE-specific regulatory element TAATGAGAT. Accumulation of IE RNA 1 and 2 was reduced four- to fivefold in in1814-infected cells, but the level of IE RNA 4 was reduced only by twofold, and IE RNA 3 was unaffected. Mutant in1814 had a high particle/PFU ratio, but many of the particles, although unable to form plaques, were capable of normal participation in the early stages of infection at high multiplicity of infection. The defect of in1814 was overcome partially by transfection of a plasmid encoding the IE protein Vmw110 into cells prior to titration and by prior infection with ultraviolet light-inactivated herpes simplex virus. Mutant in1814 was essentially avirulent when injected into mice. The results demonstrate that transinduction of IE transcription by Vmw65 is important at low multiplicity of infection and in vivo but that at high multiplicity of infection the function is redundant.