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Background

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 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. The Chromosome Determined Zinc Responsible (Czr) operon acts in exactly this manner, Czr A specifically is the regulator protein. The role of Czr A in the Czr operon is described in further detail under 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.

Biological Function

Czr A is a transcriptional repressor protein responsible for the regulation of the Czr operon. The Czr operon contains genes for the proteins Czr A and Czr B. Czr B is a Zinc transport protein which moves Zn 2+ 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 Zn 2+ 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 Zn 2+ ions, which is ideal in that this allows for expression of Czr B, a zinc transporter to be dependent on the relative amount of zinc in the cell. Czr A displays two different conformations; the first typically binds DNA and has relatively low affinity for Zn 2+, the other and has relatively low affinity for DNA.

DNA Binding

Zinc Binding

Zinc Ligand(s)

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References

1. ↑ Hanson, R. M., Prilusky, J., Renjian, Z., Nakane, T. and Sussman, J. L. (2013), JSmol and the Next-Generation Web-Based Representation of 3D Molecular Structure as Applied to Proteopedia. Isr. J. Chem., 53:207-216. doi:http://dx.doi.org/10.1002/ijch.201300024
2. ↑ Herraez A. Biomolecules in the computer: Jmol to the rescue. Biochem Mol Biol Educ. 2006 Jul;34(4):255-61. doi: 10.1002/bmb.2006.494034042644. PMID:21638687 doi:10.1002/bmb.2006.494034042644