active repressor protein no transcription
(a) no ß-galactoside sugars present—operon repressed lac I
promoter! operator lac z lac y lac a
RNA polymerase inactive repressor protein t transcription
ribosome translation t translation inducer (e.g. allolactose)
P-galactosidase permease transacetylase (b) p-galactoside sugars present—operon derepressed
Figure 6.8. Transcription of the lac operon requires the presence of an inducer.
that the operator is unoccupied by the repressor can RNA polymerase bind and generate mRNA. Thus in the absence of a fi-galactoside, only very small amounts of fi -galactosidase, fi-galactoside permease, and transacetylase are synthesized.
If lactose appears, it is converted to an isomer called allolactose. This conversion is carried out by fi-galactosidase (Fig. 6.7); as we have seen, a small amount of fi-galactosidase is made even when fi-galactoside is absent. The repressor protein has a binding site for allolactose and undergoes a conformational change when bound to this compound (Fig. 6.8ft). This means that the repressor is no longer able to bind to the operator. The way is then clear for RNA polymerase to bind to the promoter and to transcribe the operon. Thus in a short time the bacteria produce the proteins necessary for utilizing the new food source. The concentration of the substrate (lactose in this case) determines whether or not mRNA is synthesized. The lac operon is said to be under negative regulation by the repressor protein.
The transcription of the lac operon is controlled not only by the repressor protein but also by another protein, the catabolite activator protein (CAP). If both glucose and lactose are present, it is more efficient for the cell to use glucose as the carbon source because the utilization of glucose requires no new RNA and protein synthesis, all the proteins necessary being already present in the cell. Only in the absence of glucose, therefore, does E. coli transcribe the lac operon at a high rate. This control operates through an intracellular messenger molecule called cyclic adenosine monophosphate (cyclic AMP) (Fig. 6.9). When glucose concentrations are low, the concentration of cyclic AMP increases. Cyclic AMP binds to CAP, and the complex then binds to a sequence upstream of the lac operon promoter (Fig. 6.10) where it has a remarkable effect. The DNA surrounding CAP bends by about 90°. Both a subunits of RNA polymerase are now able to make contact with CAP at the same time so that the affinity of RNA polymerase for the lac promoter is increased. The result is that now the lac z, lac y, and lac a genes can be transcribed very efficiently. The lac operon is said to be under positive regulation by the CAP-cAMP complex.
To recap, the control of the lac operon is not simple. Several requirements need to be met before it can be transcribed. The repressor must not be bound to the operator, and the CAP-cyclic AMP complex and RNA polymerase must be bound to their respective DNA binding sites. These requirements are only met when glucose is absent and a j-galactoside, such as the sugar lactose, is present.
Other compounds such as isopropylthio-j-D-galactoside (IPTG) (Fig. 6.11) can bind to the repressor but are not metabolized. These gratuitous inducers are very useful in DNA research and in biotechnology. Chapter 7 deals with this and with some of the industrial applications of the lac operon.
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