The lac Operon

CAP controls a number of operons, all of which are involved in the breakdown of sugar molecules and one of which is the lac operon. When the level of the breakdown products of lactose is low, the concentration of cyclic AMP in the cell increases and CAP is switched on, binds to its specific operators, and increases the rate of transcription of adjacent operons. 

Figure 39-2. The positional relationships of the structural and regulatory genes of the lac operon. lacZ encodes 3-galactosidase,/ocT encodes a permease, and lacA encodes a thiogalactoside transacetylase. lad encodes the lac operon repressor protein.

The Lac operon (Figure 3.8, described in Esherichia coli bacteria by Jacob and Monod), illustrates how gene expression can be switched on or off according to sudden changes in environmental conditions. Glucose (a monosaccharide) is the preferred... 

The Lac operon is made up of three genes (designated A, Y and Z), which code for enzymes which metabolize lactose and the control element whose function is to activate transcription of the A,Y and Z genes. Normally (i.e. when there is sufficient glucose available), a protein called a repressor blocks the control element and so the A, Y and Z genes are off. 

The Lac operon is but one example of the genetic adaptations which allow bacteria to respond to their environment. Other examples are to be found in amino acid metabolism, for example the TRP operon which regulates tryptophan metabolism. 

Two gene regulatory proteins control the expression of the lac operon ... 

In accordance with the derivation of an expression for the regulation of the lac operon by Yagil and Yagil (83), the relationships discussed above between the relative rates of enzyme synthesis, a, and effector concentration, E, were evaluated. From... 

The well-investigated lactose operon of the bacterium Escherichial coli can be used here as an example of transcriptional control. The lac operon is a DNA sequence that is simultaneously subject to negative and positive control. The operon contains the structural genes for three proteins that are required for the utilization of lactose (one transporter and two enzymes), as well as control elements that serve to regulate the operon. 

B. The lac operon of E coli is a good model for regulation of prokaryotic gene expression in response to environmental cues (Figure 12-4). 

The lac operon has three structural genes (genes that encode protein products), the lacX, lacY, and lacK genes. 

Figure 12-4. The lac operon. A simplified version of the lac operon illustrates how activity is regulated by availability of lactose as the sole carbon source. Repressor is the product of the lad regulatory gene. Lactose in the environment is converted to allolactose, which acts as the inducer. The ON state can only occur in the absence of glucose. With repressor inactive (unbound), RNA polymerase can transcribe the structural genes.

Fig. 1.19. Tetramerization of the Lac repressor and loop formation of the DNA. The Lac repressor from E. coli binds as a dimer to the two-fold symmetric operator seqnence, whereby each of the monomers contacts a half-site of a recognition sequence. The Lac operon of E. coli possesses three operator sequences Of, 02 and 03, aU three of which are required for complete repression. Of and 03 are separated by 93 bp, and only these two sequences are displayed in the figure above. Between Of and 03 is a binding site for the CAP protein and the contact surface for the RNA polymerase. The Lac repressor acts as a tetramer. It is therefore assumed that two dimers of the repressor associate to form the active tetramer, whereby one of the two dimers is bound to 03, the other dimer binds to Of. The intervening DNA forms a so-caUed repression loop. After Lewis et al., 1996.

Several /3-galactosides structurally related to allolactose are inducers of the lac operon but are not substrates for /3-galactosidase others are substrates but not inducers. One particularly effective and nonmetaboliz-able inducer of the lac operon that is often used experimentally is isopropylthiogalactoside (IPTG) ... 

The mechanisms by which operons are regulated can vary significantly from the simple model presented in Figure 28-7. Even the lac operon is more complex than indicated here, with an activator also contributing to the overall scheme, as we shall see in Section 28.2. Before any further discussion of the layers of regulation of gene expression, however, we examine the critical molecular interactions between DNA-binding proteins (such as repressors and activators) and the DNA sequences to which they bind. 

The LexA repressor (Mr 22,700) inhibits transcription of all the SOS genes (Fig. 28-22), and induction of the SOS response requires removal of LexA. This is not a simple dissociation from DNA in response to binding of a small molecule, as in the regulation of the lac operon described above. Instead, the LexA repressor is... 

Muller-Hill, B. (1996) The lac Operon A Short History of a Genetic Paradigm, Walter de Gruyter, New York. 

Negative Regulation Describe the probable effects on gene expression in the lac operon of a mutation in (a) the lac operator that deletes most of 0 (b) the lad gene that inactivates the repressor and (c) the promoter that alters the region around position -10. 

The helix-tum-helix motif is also found in many other proteins. One of these is the bacterial lac (lactose) repressor which controls the lac operon and for which... 

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