A mutant of herpes simplex virus type 1 immediate early polypeptide Vmw175 binds to the cap site of its own promoter in vitro but fails to autoregulate in vivo

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Abstract
Vmw175, the product of herpes simplex virus type 1 immediate early (IE) gene 3, is essential for viral replication. It is required for the activation of transcription from both early and late gene promoters and also for the repression of IE gene expression. Vmw175 is able to bind specifically to certain DNA sequences, some of which (including that at the cap site of IE gene 3) contain the consensus sequence ATCGTC. The presence of this sequence at the cap site has been correlated with the ability of Vmw175 to autoregulate its own promoter. This report describes the characterization of five viruses with temperature-sensitive (ts) lesion in Vmw175. Four of these mutants express Vmw175 which is ts in its ability to bind to DNA in vitro and to autoregulate IE-3 gene expression in the infected cell. Although Vmw175 produced by the remaining mutant, ts1225, fails to autoregulate IE-3 expression at the non-permissive temperature (NPT) its DNA-binding properties are indistinguishable from those of the wild-type protein. This suggests that the ability of Vmw175 to bind to the IE-3 cap site (as measured in vitro) is insufficient for autoregulation (in vivo). All five newly characterized ts mutants are partially permissive for early gene transcription at the NPT, although Vmw175 expressed by four of them is unable to bind to the IE-3 cap site sequence at elevated temperatures. This suggests that binding to one class of recognition sequences by Vmw175, as measured in vitro, is not absolutely required for the activation of early gene promoters during virus infection. The lesions in these five ts mutants lie in the carboxyterminal third of the polypeptide; three of the mutations (those in ts1219, ts1221 and ts1225) were identified by DNA sequence analysis and were found to affect amino acid residues that are conserved in the homologous proteins from varicella-zoster virus and pseudorabies virus.