Sandbox Reserved 1053: Difference between revisions

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 allosterically 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. 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.