Thermosensitive UL9 gene function is required for early stages of herpes simplex virus type 1 DNA synthesis

DNA replication of herpes simplex virus type 1 (HSV-1) is dependent on a virus-encoded sequence-specific origin-binding protein, the product of the UL9 reading frame. We have identified the mutations in the UL9 gene of three temperature-sensitive (ts) mutants of HSV-1 which are responsible for the ts phenotype (A90T in mutant tsS and V220M in tsR and tsX). The mutations are located in two different conserved helicase sequence motifs of UL9. Two further alterations (I204T and E280D) compared to the published sequence were found in the mutant, revertant and parental wild-type strain 17syn+ sequences and therefore seemed to be irrelevant for the ts phenotype. The ts function of the UL9 protein was required at early times during DNA synthesis whereas upward temperature shifts at later times did not considerably inhibit DNA synthesis.