Sandbox Reserved 1053: Difference between revisions

<StructureSection load='2KJB' size='340' frame='true' side='right' caption='The dimer CzrA' scene=''>

The Chromosome Determined Zinc Responsible (Czr) operon acts as described above, with Czr A acting as a 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.

18177-18182.</ref>. The Czr operon contains genes for the proteins Czr A and [http://proteopedia.org/wiki/index.php/3byr Czr B]. Czr B is a Zinc transport protein which moves Zn²⁺ out of a cell while Czr A regulates this process by controlling expression level of Czr B. When relatively low amounts of zinc are present in the cell Czr A will bind to DNA, preventing the progression of RNA polymerase and thus inhibiting expression of Czr B. Decreased expression of Czr B results in the ability of the cell to retain Zn²⁺ more readily. Because Czr A and Czr B are transcribed as part of the same operon, an inhibitor of Czr A must be readily available to allow full transcription of Czr B when necessary. Czr A is noncompetitively inhibited by the binding of two Zn²⁺ ions, which is ideal in that this allows expression of Czr B, a Zn²⁺ transporter to be dependent on the relative amount of Zn²⁺ in the cell.  Czr A displays two different conformations; the first binds DNA and has relatively low affinity for Zn²⁺ (PDB code: 2kjb). In this conformation the <scene name='69/694220/A5_helices__dna_binding/1'>a5 helices are aligned</scene>. Binding of zinc drives a conformational change (PDB code: 2kjc) in which the <scene name='69/694220/A5_helices_dna_binding/1'>a5 helices become unaligned</scene>, lowering the affinity for DNA.   

Zinc acts as an inhibitor to Czr A<ref name="critical"/>, thus preventing transcriptional repression of Czr B and allowing Zn²⁺ transport out of the cell. This allows for zinc transport to essentially be self regulated. That is, when zinc concentration in the cell is relatively high, zinc ions bind to Czr A, causing a conformational change which releases the bound DNA. DNA without Czr A bound is free to be transcribed and Czr B is again expressed, allowing for Zn²⁺ transport out of the cell. At low Zn²⁺ concentrations, CzrA represses RNA Polymerase activity, and Zn²⁺ ions are maintained inside the cell.