Characterization of the TRS/IRS origin of DNA replication of herpes simplex virus type 1

In previous experiments, an origin of viral DNA replication was localized within a 995-bp DNA fragment that mapped entirely within the TRS/IRS repeated region of the herpes simplex virus type 1 genome (N. D. Stow, EMBO J. 1, 863-867, 1982). In this paper, this origin is now shown to reside within a 535-bp segment subcloned from the above fragment. Deletions extending various distances into each end of the 535-bp segment were generated using nuclease BAL 31, and the resulting plasmids screened for their ability to replicate in cells superinfected with wild-type HSV-1 helper virus. This analysis indicated that the cis-acting sequences essential for DNA replication were present within a 90-bp region, and a 100-bp viral DNA fragment containing all the signals necessary for origin function was identified. The origin lies within an untranscribed region located between the 5'-ends of two divergently transcribed immediate-early mRNAs. A prominant feature of the origin region is an almost perfect palindromic sequence 45 bp long containing 18 consecutive A or T residues at its center.