Control of ColE1 plasmid replication: binding of RNA I to RNA II and inhibition of primer formation.

Formation of the primer for ColE1 DNA replication from the primary transcript, RNA II, is regulated by an antisense transcript, RNA I. Exposure of elongating RNA II 100-360 nucleotides long to RNA I inhibits subsequent primer formation. However, primer forms normally for transcripts longer than 360 nucleotides. Therefore, the rate of binding of RNA I to RNA II is crucial to regulation. The binding rate varies among RNA II transcripts of different lengths. Transcripts longer than 200 nucleotides are bound faster than shorter ones. The Rom protein enhances binding of RNA I to these longer transcripts, whereas it suppresses binding to some shorter ones. The insensitivity to RNA I of transcripts longer than 360 nucleotides is not due to the absence of RNA I binding but to this binding having no effect on primer formation.