ColE1type

ColE1-type refers to a family of bacterial plasmid replication origins that initiate replication using an RNA primer mechanism, first described in the ColE1 plasmid of Escherichia coli. This origin is characterized by a tightly regulated system in which replication initiation depends on an RNA primer (RNA II) and an antisense RNA (RNA I) that modulates primer formation.

In ColE1-type plasmids, transcription of RNA II from the origin forms a RNA-DNA hybrid. RNase H processing creates a primer for DNA synthesis, which is carried out by the host's DNA polymerase I to generate the new strand. Replication proceeds without the bacterial chromosomal initiator DnaA, and does not rely on the chromosomal oriC. The copy number of ColE1-type plasmids is controlled mainly by RNA I, an antisense RNA that binds to RNA II and inhibits primer formation; lower RNA I levels lead to higher copy numbers, while higher RNA I levels reduce initiation events. Some ColE1 plasmids also encode the Rom (Rop) protein, which stabilizes the RNA I–RNA II interaction and thereby influences copy number.

ColE1-type origins are widely used in molecular cloning vectors due to their controllable copy number and compatibility with common bacterial hosts. Examples of vectors with ColE1-type origins include pBR322-derived plasmids (medium copy) and pUC-derived plasmids (high copy). The system provides a compact, well-characterized means of regulating replication in plasmid engineering and biotechnology.