= Colicin E1 plasmid
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The \b[Colicin E1 plasmid], usually called ColE1, is a <plasmid> associated with the antibacterial <protein> <Colicin E1> in <Escherichia coli>. It is a circular <DNA> replicon studied both as a biological system and as a source of replication-control elements for engineered <plasmids>. A vector carrying a ColE1-derived replication region need not carry all the genes or properties of the natural <plasmid>.
Its replication provides an illuminating role for functional <RNA>. A transcript, <ColE1 RNA II>, forms an RNA–DNA hybrid near the replication start. Processing by <Ribonuclease H> generates an <RNA primer> that can support <DNA replication>. A second transcript, <ColE1 RNA I>, binds the primer transcript and inhibits formation of the persistent hybrid, thereby preventing primer production. RNA pairing can therefore regulate whether <DNA replication> starts.<Control of ColE1 plasmid replication: the process of binding of RNA I to the primer transcript by J. Tomizawa>[Abstract: primer-promoter transcript, hybrid processing and inhibition by RNA I.]{ref}
The experiments identified the RNA-binding product rather than inferring regulation solely from changes in plasmid abundance. The mechanistic distinction is useful: a perturbation that changes <ColE1 RNA I>–<ColE1 RNA II> interaction acts at an initiation-control step, whereas a perturbation of DNA-chain elongation acts later. Either can change the final amount of <plasmid> DNA, so endpoint abundance alone does not locate the defect.<Control of ColE1 plasmid replication: the process of binding of RNA I to the primer transcript by J. Tomizawa>[Binding-product identification and inhibition of primer formation.]{ref}
ColE1 also served as a substrate in early <genetic recombination> experiments. Electron microscopy of plasmid recombination intermediates supplied structural evidence about how DNA molecules can exchange strands, showing how a compact bacterial replicon can make a general molecular process experimentally accessible.<On the mechanism of genetic recombination: electron microscopic observation of recombination intermediates by H. Potter and D. Dressler>[Primary electron-microscopic observation of ColE1 recombination intermediates.]{ref}
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