Iterons are directly repeated

DNA sequence DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...

s which play an important role in regulation of

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...

copy number in

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

. It is one among the three negative regulatory elements found in plasmids which control its copy number. The others include

antisense RNA Antisense RNA (asRNA), also referred to as antisense transcript, natural antisense transcript (NAT) or antisense oligonucleotide, is a single stranded RNA that is complementary to a protein coding messenger RNA (mRNA) with which it hybridizes, an ...

s and ctRNAs. Iterons complex with cognate replication (Rep) initiator

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...

s to achieve the required regulatory effect.

Regulation of Replication

Iterons have an important role in plasmid replication. An iteron-containing plasmid

origin of replication The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by se ...

can be found containing about five iterons about 20

base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both D ...

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.

Limiting Factors of Initiation

There are 4 main limiting factors leading to no initiation of replication in iterons: Screen Shot 2015-12-02 at 6

*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

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

s of these proteins are no longer in a high enough concentration to initiate replication. Another limiting factor, titration, occurs after replication and works to prevent saturation by distributing monomers to daughter origins so that no are fully saturated. Finally, handcuffing refers to pairing origins leading to inactivation. This is mediated by monomers and inactivation is due to

steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

between the origins. 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 it has been shown this can decrease plasmid copy number. In contrast, removing these extra iterons will increase copy number.

Replicon Structure

Plasmids are known to have very similar structure when under control of Iterons. This structure consists of an origin of replication upstream of a gene that codes for a replication initiator protein. The iterons themselves are known to cover about half of the origin of replication. Usually, iterons on the same plasmid are highly conserved, whereas comparing iterons on different plasmids still exhibit homology yet are not as highly conserved. This suggests that iterons could be

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

arily related.

Replication Initiator Proteins

The replication initiator protein (Rep) plays a key role in initiation of replication in plasmids. In its monomer form, Rep binds an iteron and promotes replication. The protein itself is known to contain two independent

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

and

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...

globular domains that subsequently bind to two domains of the iteron. The dimer version of the protein is generally inactive in iteron binding, however it is known to bind to the repE

operator Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another ...

. This operator contains half of the iteron sequence making it able to bind the dimer and promote gene expression. Plasmids containing iterons are all organized very similarly in structure. The gene for Rep proteins is usually found directly downstream of the origin of replication. This means that the iterons themselves are known to regulate the synthesis of the rep proteins.

References

{{reflist Genetics techniques Molecular biology