ColE1

ColE1 is a plasmid found in bacteria. Its name derives from the fact that it carries a gene for colicin E1 (the cea gene). It also codes for immunity from this product with the imm gene. In addition the plasmid has a series of mobility (mob) genes.

Replication

ColE1 replication begins at the oriV origin. 555bp upstream from this point, RNA polymerase initiated transcription of RNAII which acts as a pre-primer. The transcript folds into a secondary structure which stabilises the interaction between the nascent RNA and the origin's DNA. This hybrid is attached by RNase H, which cleaves the RNA strand exposing a 3' hydroxyl group. This allows the extension of the leading strand by DNA Polymerase I. Laggind strand synthesis is later initiated by a primase encoded by the host cell.

Copy number control

RNAI is a counter-transcript to a section of RNAII and so binds to its 5' end. This alters the folding of RNAII so that the DNA-RNA hybrid is not stabilised and cleavage does not occur. This ensures that at high copy numbers (and so high RNAI concentration), replication is slowed down. Rom is a secondary replication repressor, it stabilises the RNAI-RNAII hybrid and stabilises it. Rom may be especially important at preventing runaway replication at slow growth rates.