A PARTICULAR DNA-STRUCTURE IS REQUIRED FOR THE FUNCTION OF A CIS-ACTING COMPONENT OF THE EPSTEIN-BARR-VIRUS ORILYT ORIGIN OF REPLICATION

OriLyt, the cis-acting element of Epstein-Barr virus lytic origin of r eplication, consists of upstream and downstream components. The upstre am component plays a dual role in transcription and replication. The d own-stream component contains a homopurine-homopyrimidine sequence whi ch forms an H palindrome, We show that the downstream component can ad opt a triple helix structure in vitro,that the 5' border of the homopy rimidine sequence is sensitive to P1 nuclease when carried by a superc oiled plasmid and that an oligonucleotide complementary to the homopyr imidine strand is taken up by a plasmid carrying the OriLyt H palindro me. We also show that all mutations which alter the H palindrome impai r both oligonucleotide uptake and OriLyt-dependent replication. Intere stingly, compensatory mutations which restore an H palindrome also res tore oligonucleotide uptake by the mutated plasmids and their OriLyt-d ependent replication. Thus, there is a strong correlation between the inability of the OriLyt H palindrome to form a non-B-DNA structure in vitro and impairment of OriLyt-dependent replication. This suggests th at the presence of a non-B-DNA structure in the OriLyt downstream comp onent is required for OriLyt-dependent replication.