Escherichia coli (bacteria)
"Lactose intolerance" is the inability to digest lactose, the predominant sugar of milk, and its common symptoms include nausea, cramps, bloating, gas, and diarrhea. The inability results from a shortage of the enzyme lactase, which breaks down lactose into simpler sugars that then can be absorbed into the bloodstream. In 1961, F. Jacob and J. L. Monod proposed a theory about the genetic regulatory system, "the operon theory", based on the study of the lactose operon of Escherichia coli. The lac operon is a series of genes related to lactose metabolism and consists of structural genes, e.g. lactase gene, and regulatory genes which control the synthesis of enzyme proteins. When the repressor, a product of regulatory gene, binds to the operator DNA which controls transcription of structural genes depending on the amount of lactose present, the transcription is inhibited. In contrast, when lactose as an inducer binds to the repressor, the repressor releases DNA and then the repression of transcription is removed. Therefore the repressor acts as the negative control factor for transcription. RNA polymerase binds to the promoter DNA, the upstream region of the operator, and begins the transcription. In this region, cyclic AMP receptor protein (CRT-cAMP) acts as a positive control factor in a similar fashion.
More than 30 years after, three structures of the repressor monomer, the repressor bound to the inducer and the complex of the repressor and operator DNA were determined, and the mechanisms and conformations for promoting and repressing of transcription were elucidated. The lac repressor is a protein of 360 amino acids that associates into a homotetramer. The repressor monomer has four functional units, an NH2-terminal headpiece which binds specifically to operator DNA, a hinge region, an inducer binding domain, and a COOH-terminal helix. The inducer binding domain consists of two subdomains that are topologically similar, and binds to an inducer between the two subdomains. An oligomerization domain is formed when all four COOH-terminal helices of the tetramer associate. The repressor tetramer is bound to two independent, symmetric operator DNA. In the inducer binding state, the NH2-terminal core subdomains contain some electrostatic interactions that stabilize the dimer formations, and N-terminal headpieces and hinge helices are probably mobile. On the other hand, in the DNA binding state, the hinge regions form helices, and the NH2-terminal core subdomains translate and rotate and then the interaction between subdomains are broken. Moreover the COOH-terminal oligomerization helices change conformation. Thus, comparing the structures of the repressor bound to inducers and the complex of repressor and operator DNA suggests the mechanisms of the induced and repressed states of transcription.
Protein Data Bank (PDB)
Lewis, M. Chang, G. Horton, N.C. Kercher, M.A. Pace, H.C. Schumacher, M.A. Brennan, R.G. Lu, P.; "Crystal structure of the lactose operon repressor and its complexes with DNA and inducer."; Science; (1996) 271:1247-1254 PubMed:8638105.
author: Yuko Tsuchiya