Repressible operons

Fig. 16.6. A repressible operon. The repressor is inactive until a small molecule, the corepressor, binds to it. The repressor-corepressor complex binds to the operator and prevents transcription.

Sinha, S. R. Ramaswamy. 1988. Complex behaviour of the repressible operon. J. Theor. Biol. 132 307-18. 

Glucose is metabolized preferentially because it results in a decrease in the synthesis of cAMP by adenylate cyclase. The lack of cAMP prevents the formation of the cAMP-CAP complex, which is necessary for the efficient transcription of the lac operon and other catabolite-repressible operons. 

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.

FIGURE 28-5 Representative prokaryotic operon. Genes A, B, and C are transcribed on one polycistronic mRNA. Typical regulatory sequences include binding sites for proteins that either activate or repress transcription from the promoter. 

Despite this elaborate binding complex, repression is not absolute. Binding of the Lac repressor reduces the rate of transcription initiation by a factor of 10s. If the 02 and 03 sites are eliminated by deletion or mutation, the binding of repressor to CL alone reduces transcription by a factor of about 102. Even in the repressed state, each cell has a few molecules of /3-galactosidase and galactoside permease, presumably synthesized on the rare occasions when the repressor transiently dissociates from the operators. This basal level of transcription is essential to operon regulation. 

Repression and induction. Immediately adjacent to the promoter is the operator (O), which is a binding site for a repressor (R). When the operator is free, transcription is initiated and proceeds through the operator region and on to the genes coding for the three proteins. On the other hand, if the repressor is bound to the operator, transcription is blocked. When the operon model was first proposed, the chemical nature of the repressor was unknown. However,... 

The lac operon is ordinarily subject to repression and is activated by the presence of an inducer, now known to be allolactose, D-Galp-(il— 6-d-G1c. However, in experimental work artificial inducers such as isopropyl-[5-o-thiogaIactoside (IPTG) are most often used. Jacob and Monod postulated that the free repressor protein binds to the operator. In the presence of the inducer a conformational change takes place, destroying the affinity of the repressor protein for the operator site. Thus, in the presence of inducer the operator is not blocked, and the transcription takes place. Such an operon is said to be negatively controlled and inducible. 

Repressors may have similar recognition domains but may vary greatly both in size and in the functioning of their other domains, which may react both with small allosteric effectors and with other proteins. The repressor BirA of the E. coli biotin synthesis operon is an enzyme. The 321-residue protein activates biotin to form biotinyl 5 -adenylate and transfers the biotinyl group to proteins such as acetyl-CoA carboxylase107-1093 and also represses transcription. 

Control by looping. The arabinose utilization operon of E. coli, araBAD, encodes proteins needed for uptake of arabinose and conversion to D-xylulose 5-P. The repressor AraC in the absence of arabinose binds at operator 1 (Oj) to prevent further synthesis of repressor (autorepression) and also at the aral region to repress transcription of operon araBAD. The operator 2 (02) site, which is 211 bp upstream from aral, is also needed for this repression.145-14713 A loop is apparently formed by repressor binding (Fig. 28-7). Binding of arabinose to the repressor converts it into an activator, which stimulates initiation of transcription at the BAD promoter. Further stimulation is provided by the CAP-cAMP complex, which binds next at aral. 

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