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A bacteriocin is an antimicrobial peptide or protein produced by bacteria that acts against susceptible bacterial strains. The category describes a biological product and its antibacterial action; susceptibility depends on both the product and the target.^[ref]

Colicin E1 supplies a concrete mechanistic example. An experiment compared the complete protein with its C-terminus fragment in artificial lipid bilayers. Similar channel properties in the two forms localized pore-forming activity to that region. Membrane association could occur without the membrane potential needed for conducting channels to form.^[ref]

The experiment shows how to separate delivery, attachment and action of a bacterial product. It establishes a channel mechanism for the tested Colicin E1 system; the class name bacteriocin does not assign that same mechanism to every member. A growth-inhibition observation alone would not identify which molecular step caused the inhibition.

Colicin E1 is a pore-forming bacteriocin, a bacterial protein capable of disrupting susceptible bacterial cell membranes. Its channel-forming region lies toward the C-terminus. The protein is distinct from the Colicin E1 plasmid, the DNA element associated with its production.

A 1983 experiment compared intact Colicin E1 with its roughly $20\mathrm{kDa}$ terminal fragment in artificial planar lipid bilayers. Both produced similar individual-channel and aggregate electrical behaviour, supporting localization of channel-forming function to that part of the protein.^[ref]

Both forms could associate with a lipid bilayer without an applied electric potential difference, while formation of conducting channels required the specified polarity of membrane potential. Attachment and channel opening are therefore distinguishable steps: seeing membrane-associated protein does not establish that an ion-conducting pore is open.^[ref]

The 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.^[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.^[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.^[ref]

J. O. Bullock studied how Colicin E1 forms ion-conducting channels in lipid bilayers. A 1983 study compared the complete protein with a fragment from its C-terminus, measuring both individual-channel and aggregate electrical behaviour.^[ref]

The fragment retained properties very similar to those of the complete Colicin E1. Both forms could bind a lipid bilayer before a suitable membrane potential permitted conduction. Fragmentation therefore helped locate a functional region, while electrical measurements distinguished attachment from pore opening. These are separate experimental questions even though both involve the same protein at the same membrane.^[ref]