Two sites within the short region origin of DNA replication (ori) in herpes simplex virus type 1 (HSV-1) which bind the product of the UL9 gene have previously been identified. One of these sites (site I) contains an 11 bp sequence which is also present in ori of varicella-zoster virus, and the other (site II) includes a related element differing in two positions. A third sequence (motif III), which lies close to binding site I, differs from the site I element at only a single position. We have deleted specifically each of these three 11 bp sequences from within functional copies of HSV-1 ori and have examined the effects on origin activity and binding of the UL9 protein. Gel retardation analyses confirmed the important roles of the regions deleted from sites I and II in interacting with the UL9 protein. In transient replication assays, copies of ori lacking the site I or II elements exhibited undetectable or residual (4 to 8%) activity respectively. The UL9 protein did not bind to motif III even in the absence of site I sequences, although removal of the motif III sequence caused a small reduction in ori activity. A single base change which converted the sequence within binding site I to that of motif III was sufficient to abolish both the interaction of the UL9 gene product at this locus and the replicative ability of ori. Therefore, interaction of the UL9 protein with binding site I is essential for origin activity, but the presence of binding site II is also required for efficient replication.


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