Necessary for the bidirectional replication of DNA. It interacts with the ori (origin of replication) region of the genome during the initiation of replication. Belongs to the phage O protein family. The amino end of replication protein O may be as shown or may begin at residue 22. There is a UGA terminator after residue 299. In vitro, this terminator codon is skipped and read-through occurs. It is not known whether read-through occurs in vivo.