Iteron
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Iterons are directly repeated DNA sequences which play an important role in regulation of plasmid copy number in bacterial cells. It is one among the three negative regulatory elements found in plasmids which control its copy number. The others are antisense RNAs and ctRNAs. Iterons complex with cognate replication (Rep) initiator proteins to achieve the required regulatory effect.[1][2]
Limiting factors of initiation
Iterons have an important role in plasmid replication. An iteron-containing plasmid origin of replication can be found containing about five iterons about 20 base pairs in length total. These iterons provide a saturation site for initiator receptor proteins and promote replication, thus increasing plasmid copy number in a given cell.[1]
There are four main limiting factors leading to no initiation of replication in iterons:[1]
- Transcriptional autorepression
- Initiator dimerization
- Initiator titration
- Handcuffing
Transcriptional auto-repression is thought to reduce initiator synthesis by repressing the formation of the Rep proteins. Since these proteins work to promote binding of replication machinery, replication can be halted in this form. Another factor used to stop replication is known as dimerization. It works to dimerize these Rep proteins, and as a result, monomers of these proteins are no longer in a high enough concentration to initiate replication.[2] Another limiting factor, titration, occurs after replication, and works to prevent saturation by distributing monomers to daughter origins so that none are fully saturated. Finally, handcuffing refers to pairing origins leading to inactivation. This is mediated by monomers, and inactivation is due to steric hindrance between the origins.[1][2]
Another less prevalent limitation thought to be present in these iterons is the presence of extra repeats. If a plasmid contains an extra supply of iterons outside of the saturation site, this can decrease plasmid copy number. In contrast, removing these extra iterons will increase copy number.[1]
