Lac operon, in E. coli

Why is control of expression of the lac operon in E. coli said to be an example of negative control ... 

Functional map of the lac operon in E. coli. Note that the lacl gene is physically linked to the lac operon, but it is not considered to be part of the lac operon because the lacl promoter is not regulated by factors that... 

The operator of the lac operon in E. coli (Fig. 94) is composed of four sites — each of them five nucleotides in length — which are separated by one-two nucleotide spacers. The synthesis of repressor proteins is controlled by a special gene known as the regulator (i). The protein repressor for each operator is constantly being synthesized, but at a very low rate. No more than 10-20 molecules are present in the cell at any time. The attachment of the activated operon protein (CAP), which is an acceptor of cAMP, and the removal of repressor protein from the DNA of the operator under the control of substrate (lactose) concentration, are both necessary for the initiation of transcription. Cyclic AMP does not associate with the activator (CAP) if its concentration is low in the cell. Hence, the operon remains in the repressed state (Blattner and Dahlberg, 1972 Blattner et al., 1972 Khesin, 1972 Bresler, J973). The termination of transcription is also determined by a special site on the DNA. 

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). 

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 28-1 Schematic representation of the lac operon of E. coli and of its control. Here only the template strand of the DNA is shown. However, the coding (nontranscribed) strand is usually the one labeled, as in Fig. 28-2.

The answer is d. (Murray, pp 468-487. Scriver, pp 3-45. Sack, pp 245-257. Wilson, pp 151-180.) Several operons in E. coli, including the lac operon, are subject to catabolite repression. In the presence of glucose, there is decreased manufacture of cyclic AMP (cAMP) by adenylate cyclase. Low glucose levels increase production of cAMP, which binds to the catabolite activator protein (CAP). The cAMP-CAP complex binds to the promoters of several responsive operons at catabolite activator protein... 

The arabinose (ara) operon in E. coli utilizes some of the same regulatory principles as the lac operon, but three additional mechanisms are also found. First, in the ara operon, the expression of a transcription factor gene is autoregulated by it s own gene product (AraC controls araC expression). Second, a transcription factor can function as a repressor or an activator in an effector-dependent manner (arabinose changes AraC from a repressor into an activator). Third, transcription factors often function together to effect a change in transcriptional initiation (AraC and CAP must both be bound to fully induce the ara operon). 

It has been known since 1899 that some enzymes in bacteria are formed only in the presence of specific substrates [92]. In 1946 Monod and Audureau [3] demonstrated that such adaptive enzyme systems were under the control of specific genetic determinants. Since then a large number of inducible systems have been discovered and studied in bacteria, particularly in Escherichia coli and Salmonella. In the case of lac operon of E. coli the depth of our understanding is probably greatest. 

The current concept of a classic operon is largely derived from studies of the lac operon of E. coli and includes a promoter at the beginning which is considered to be the site at which the DNA-dependent RNA polymerase initiates transcription [151,152]. The promoter is followed by a closely associated operator serving as the site of attachment for repressor molecules which can prevent or impede transcription [153]. The structural genes are next in the sequence, and in the case of the tryptophan operon they code for the peptides which make up the tryptophan biosynthetic enzymes. Presumably there is an element at the end of the operon which acts as a signal to terminate transcription. This section will describe information derived primarily from work with E. coli and S. typhimurium. A number of general reviews and discussions of regulation have been published recently [4-6,105,154]. 

---