A 67-base-pair segment from the Ori-S region of herpes simplex virus type 1 encodes origin function

The Ori-S segment of herpes simplex virus type 1 contains a 45-base-pair-long imperfect palindrome with an AT segment at its center. We cloned Ori-S into a poisonless plasmid to investigate the role of the palindromic components in DNA replication. Neither a large insertion within the AT segment nor a deletion of the right side of the palindrome significantly inhibited DNA replication under our conditions of analysis. These findings argue against the necessity for a specific cruciform structure in the initiation of replication. We scanned the entire AT segment with triple tandem-base-pair substitutions to pinpoint essential functional sequences. Only the first 3 base pairs at the left end of the segment are absolutely essential for replication in the presence of the remaining AT sequences.