Tryptophan operon repressor/operator complex
Escherichia coli (bacteria)
In 1965 three French scientists, Jacob, Lwoff and Monod, were awarded the Nobel prize in medicine for explaining how cells control the production of proteins at the genetic level. As we know the information for making new proteins is contained in the DNA. But what decides and when is it decided to make them? Some of the genes for proteins are always turned on. They will produce new proteins uncontrolled by external events. Other genes will be expressed as a result of external events. The hypothesis that Jacob, Lwoff and Monod postulated was that expression of the genes was controlled by proteins. They called the system the operon. The operon consists of two things: a gene or set of genes that may or may not be expressed and a protein that binds to the DNA at a specific sequence near these genes. When the protein is bound to the DNA, expression of the gene(s) is blocked, when the protein is not bound, expression proceeds. The example here is such an operon. The protein itself is called the operator or repressor, repressing expression of the gene.
The protein is a dimer and the PDB structure shows two such dimers attached to a long strand of DNA. The two monomers of each dimer are locked into each other so closely that it is almost difficult to see which one is which. Each monomer has a binding site for the amino acid tryptophan. Hence the operator is called trp repressor (trp for tryptophan). When tryptophan is present in the cell, the cell does not need to make tryptophan from other sources. The expression of the genes for protein synthesizing tryptophan is then switched off. Some of the tryptophan present in the cell binds to the trp repressor and it stays bound to the DNA. This is the case in the example shown here. Each monomer contains one molecule of tryptophan bound to it and the protein is therefore attached to the DNA.
Protein Data Bank (PDB)
Otwinowski, Z. Schevitz, R.W. Zhang, R.G. Lawson, C.L. Joachimiak, A. Marmorstein, R.Q. Luisi, B.F. Sigler, P.B.; "Crystal structure of trp repressor/operator complex at atomic resolution."; Nature; (1988) 335:321-329 PubMed:3419502.
author: Arno Paehler