Lac-operon

More than 30 years ago Jacob and Monod introduced the Escherichia coli lac operon as a model for gene regulation. The lac repressor molecule functions as a switch, regulated by inducer molecules, which controls the synthesis of enzymes necessary for E. coli to use lactose as an energy source. In the absence of lactose the repressor binds tightly to the operator DNA preventing the synthesis of these enzymes. Conversely when lactose is present, the repressor dissociates from the operator, allowing transcription of the 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.

RNA Polymerase (a) How long would it take for the E. coli RNA polymerase to synthesize the primary transcript for the E. coli genes encoding the enzymes for lactose metabolism (the 5,300 bp lac operon, considered in Chapter 28) (b) How far along the DNA would the transcription bubble formed by RNA polymerase move in 10 seconds ... 

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. 

In bacteria, genes that encode products with interdependent functions are often clustered in an operon, a single transcriptional unit. Transcription of the genes is generally blocked by binding of a specific repressor protein at a DNA site called an operator. Dissociation of the repressor from the operator is mediated by a specific small molecule, an inducer. These principles were first elucidated in studies of the lactose (lac) operon. The Lac repressor dissociates from the lac operator when the repressor binds to its inducer, allolactose. 

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