Characterization of a Herpes Simplex Virus type 1 Deletion Variant (1703) Which Under-produces Vmw63 During Immediate Early Conditions of Infection

MRC Virology Unit, Institute of Virology, Church Street, Glasgow G11 5JR, U.K. The herpes simplex virus type 1 deletion variant 1703 apparently fails to synthesize the essential IE2 gene product Vmw63 despite the deletion leaving the gene intact. Sequence analysis revealed that the deletion removes a region to the right of IE2 comprising the 3' end of IE1, UL56 and the 3' part of UL55, stopping 555 bp downstream of the IE2 polyadenylation signal. Further DNA sequencing has shown that there is no secondary mutation in the IE2 gene. Western blot analysis demonstrated that Vmw63 is made at reduced levels compared to that produced by the wild-type virus during immediate early conditions of infection. S1 nuclease protection mapping has revealed that this reduction is also apparent at the level of mRNA synthesis. A direct link between the deletion and the change in mRNA synthesis was provided by the insertion of a deletion-spanning fragment from 1703 into a 17 + genome, which resulted in the recombinant having a 1703-like phenotype. Evidence that down-regulation of IE2 mRNA during immediate early conditions of infection could be due to antisense RNA initiating from the IE1 promoter was obtained by the insertion of a novel transcriptional termination signal between IE1 and IE2 in the variant and the subsequent detection of wild-type levels of IE2 mRNA and protein. Present address: Department of Microbiology, The University, Glasgow G12 8QQ, U.K. Received 1 November 1993; accepted 16 December 1993.