English: The copy number of plasmids with the ori of ColE1 is regulated by two RNA transcripts, RNA II ad RNA I. RNA II is needed for the initiation of replication, whereas RNA I acts as its repressor.

The nascent RNA II, whose transcription is initiated 555 bp upstream of the replication origin, forms a hybrid with the template DNA near the ori. RNase H cleaves the RNA strand and exposes a 3' hydroxyl group. The processed RNA II can then serve as a primer for DNA synthesis by DNA Polymerase I. The primer formation is inhibited by RNA I, a shorter transcript which is complementary to the 5` end of RNA II. When RNA I is present, it forms a hybrid with RNA II. This alters the folding of RNA II so that the DNA-RNA hybrid is not stabilized and cleavage does not occur.

The binding of RNA I to RNA II is enhanced by the rop protein.