Figure 1. The lac structural genes.

Figure 2. Three-dimensional structure of -galactosidase. (a) Ribbon representation of the -galactosidase tetramer showing the largest face of the molecule. Contacts between red/green and blue/yellow dimers form the long interface. Contacts between the red/yellow and blue/green dimers form the activating interface. Formation of the tetrameric particle results in two deep clefts that run across opposite faces of the molecule. Each contain two active sites. (b) Ribbon diagram of the blue/green dimer viewed down the molecular 2-fold axis, showing the composition of the activating interface. Residues 1–50 from each chain, which form the -complementation region (see text), are shown in red. The interface includes contacts between the respective complementation peptides, between two helices from the respective monomers that pack together to form a four-helix bundle, and between an extended loop (residues 272–288) from each monomer that reaches across the interface and extends into the active site region of the neighbouring monomer stabilizing the active site structure. (c) Stereo ribbon diagram of the -galactosidase monomer showing the domain organization of the chain. Residues corresponding to successive domains are coloured in successive spectral colours. Reprinted from Jacobson RH, Zhang XJ, DuBose RF and Matthews BW (1994) Three-dimensional structure of -galactosidase of E. coli. Nature 369;761–766, with permission.

Figure 3. The ‘PaJaMo’ experiment. A male (Hfr) lac i⁺z⁺ strain was conjugated with a female (F^–) lac i^–z^– strain in the absence of inducer. At the time indicated, inducer was added to one of the cultures, whereas the other one received no addition. -Galactosidase activity was measured as a function of time. Adapted from Pardee AB, Jacob F and Monod J (1959) The genetic control and cytoplasmic expression of ‘inducibility’ in the synthesis of -galactosidase by E. coli. Journal of Molecular Biology 1; 165–178, with permission.

Figure 4. The crystal structure of the Lac repressor–DNA complex constructed from the available Protein Data Bank structures (Lewis et al., 1996) by modelling procedures (Balaeff et al., 2004). In the V-shaped tetrameric Lac repressor each of the two dimers (drawn as purple protein cartoon) binds with high specificity to a 21-base pair operator DNA fragment (drawn as blue tubes and red spheres) through the N-terminal 62 residue-long headpiece (drawn in green). Dimer–dimer assembly occurs via a compact four-helical bundle formed by 18 C-terminal residues from each subunit (drawn as orange tubes). Adapted from Balaeff A, Mahadevan L and Schulten K (2004) Structural basis for cooperative DNA binding by CAP and Lac repressor. Structure 12; 123–132, with permission from Elsevier.

Figure 5. Diagram of the lactose operon in the repressed (a) and induced (b) states. Synthesis of the lactose operon proteins, genetically determined by the structural genes (lacZ, lacY and lacA), is blocked by the LacI repressor synthesized by the regulator gene, lacI. The operator (O) is the site of specific interaction with the repressor. The repressor can be inactivated by the inducer, thus allowing transcription to take place at the promoter. Inherent to the operon model is the assumption that transfer of genetic information from gene to protein involves a short-lived mRNA. Reprinted with permission from Jacob F and Monod J (1961) Genetic regulatory mechanisms in the synthesis of proteins. Journal of Molecular Biology 3: 318–356. Copyright © 1961 Academic Press. Drawing courtesy of Jean-Marc Ghigo.

Figure 6. Schematic structure of the CAP–DNA complex. For the DNA, sharply bent by CAP, the bases and backbone are shown in red. The protein is represented as a ribbon diagram (blue) with two cAMP molecules (red) placed to indicate the binding sites on each subunit of CAP. Reprinted from Parkinson G, Gunasekera A Vojtechovsky J et al. (1996) Aromatic hydrogen bond in sequence-specific protein DNA recognition. Nature Structural Biology 3: 837–841, with permission from Nature Publishing Group. Figure courtesy of Richard H. Ebright.