Sandbox Reserved 1053: Difference between revisions

===Operon Overview===

[https://en.wikipedia.org/wiki/Operon Operons] are a critical genetic component of most prokaryotic cells. There are many different operons that are responsible for the production of proteins with a wide range of functions, the most well-known of which are the Lac and Trp operons, responsible for producing enzymes which metabolize lactose and tryptophan respectively. Despite many differences in each operon and the proteins that they encode, operons all function in the same general manner. Structurally, each operon contains a regulator, an operator, and one or more structural genes. The regulator protein is responsible for managing the expression level of the structural genes, the operator is similar to a promoter in a regular gene and is where transcription begins, and the structural genes code for proteins. The regulator protein (produced as a result of expression of the regulator gene) most often acts in a repressive manner, though this is not always the case. That is, the regulator protein will bind to the operator of the operon, inhibiting the binding and/or progression of [https://en.wikipedia.org/wiki/RNA_polymerase RNA polymerase] to the structural genes, thus inhibiting transcription of the genes into mRNA.  If the regulator protein were to consistently be active, there could never be adequate expression of the structural genes, so there must be a way to inactive the regulator protein, thus enabling expression of the structural genes. This is achieved through the binding of an inhibitor to the regulator protein. Since regulator proteins are DNA binding proteins, often this inhibition is allosteric rather than competitive, that is the inhibitor is not something that mimics DNA and binds to the active site physically blocking DNA from binding. Rather, the inhibitor of the regulator binds to somewhere other than the active site of the protein, changing it in some way which decreases the proteins affinity or ability to bind DNA.  

===Czr Operon===

The Chromosome Determined Zinc Responsible (Czr) operon acts as described above, with Czr A acting as the regulator protein. As such, Czr A is responsible for controlling the transcription of the rest of the operon and by extension, the transport of Zn ²⁺ out of the cell; the role of Czr A in the Czr operon is described in further detail as part of the explanation of biological function. In addition to being a component of an operon, Czr A is also considered to be a metal sensor protein. While the immediate function of Czr A is gene regulation, this serves the larger purpose of acting to maintain an appropriate concentration of Zn ²⁺ in the cell.

== Structural Overview ==

CzrA functions as a [https://en.wikipedia.org/wiki/Dimer_(chemistry) dimer]. The <scene name='69/694218/Monomeric_unit/1'>monomeric units</scene> dimerize at the czr operon, repressing gene transcription. Each monomeric unit contains <scene name='69/694218/Helices/1'>five alpha helices</scene> seen in purple and <scene name='69/694218/B_sheets/1'>two beta sheets</scene> displayed in yellow. While the function of the [https://en.wikipedia.org/wiki/Beta_sheet beta sheets] are not yet known, key [https://en.wikipedia.org/wiki/Alpha_helix helices] regulate the binding of DNA and Zn⁺². The <scene name='69/694218/Alpha_4_helix/1'>alpha 4 helix</scene> is the location of DNA binding and the <scene name='69/694218/Alpha_5_helix/1'>alpha 5 helix</scene> contains the Zn⁺² binding site. As Zn⁺² binds, the alpha 4 helices are <scene name='69/694218/Alpha_4_helices_pushed/1'>pushed out of alignment</scene>, repressing their DNA binding ability.

== Binding of DNA ==

   

Zinc⁺² binding is driven by a large [https://en.wikipedia.org/wiki/Entropy entropic] gain <ref>DOI:10.1021/ja906131b</ref>. Water molecules around the metal ion and CzrA protein are displaced, and gain greater freedom. This gain in entropy allows Zn⁺² to bind to CzrA with reasonable affinity and speed in vivo. The zinc⁺² ion forms a tetrahedral complex with the four residues (Figure 1). This allows other metal ions to act as allosteric inhibitors to CzrA. Any metal that may form a tetrahedral complex will have some affinity for CzrA, assuming it is not too large to fit into the pocket. However, the metal binding pocket of CzrA has been optimized  to bind Zn⁺² with the highest affinity. As CzrA is a transcriptional repressor, binding of Zn⁺² to the dimer will activate the czr operon. Zn⁺² is preferred as CzrB opens a Zn⁺² channel, allowing the excess zinc ions to export the cell.  

== References ==

<references/>