Lactose operon, in E. coli

The end products of gene expression are proteins, mainly enzymes, and it is essential that their levels be strictly controlled. There are many potential sites of control in both bacteria and eukaryotes. DNA or gene amplification (Chap. 16) in eukaryotes is one way of responding to the demand for more of the protein product if there arc more copies of the gene, then transcription can occur at a faster rate. More often, control is effected at the level of cither transcription or translation, with the former probably being more important for both bacteria and eukaryotes. Transcriptional control in bacteria is particularly effective because of the very short half-life (a few minutes) of mRNA in such cells the half-life is longer in eukaryotes. The prototype for transcriptional control is the lactose operon in E. coli. 

The three enzymes of the lactose operon in E. coli are not produced in precisely equimolar amounts follotving induction. Rather, more galactosidase than permease is produced, and more permease than transacetylase is produced. Propose a mechanism to account for this that is consistent with known facts about the lactose operon. 

Transcriptional control in bacteria is particularly effective because of the very short half life (a few minutes) of mRNA in such cells it is longer in eukaryotes. The prototype for transcriptional control is the lactose operon in E. coli. 

Many attempts have been made to demonstrate that hormones exert their effects on gene expression at the level of gene transcription, analogous to the action of inducers on the lactose operon in E. coli [19]. 

Table 2-6. Structural genes in the lactose operon in E. coli...

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. 

Schwartz, D., Beckwith, J. R. Mutants missing a factor necessary for the expression of catabolite-sensitive operons in E. coli. In J. R. Beckwith and D. Zipser (eds.). The lactose operon, p. 417-422. Cold Spring Harbor Laboratory 1970. 

CAP and cAMP activate a large number of genes in E. coli that are concerned with catabolism. When glucose is present, the cAMP is greatly lowered and the lac operon is expressed at a very low level, even when lactose is present. This is because glucose is a more readily metabolizable carbon source than lactose. 

Fig. 4. Selection against the strain of E. coli carrying Tn 10 in the lactose operon in a glucose-limited chemostat.

In E. coli, about half the genes are clustered into oper-ons each of which encodes enzymes involved in a particular metabolic pathway or proteins that interact to form one multisubunit protein. For instance, the trp operon mentioned earlier encodes five enzymes needed in the biosynthesis of tryptophan (see Figure 4-12). Similarly, the lac operon encodes three enzymes required for the metabolism of lactose, a sugar present in milk. Since a bacterial operon is tran-... 

The catabolite activator protein is a transcription factor in E. coli that stimulates transcription of the lac operon structural genes. It responds to cAMP levels such that the lac operon is transcribed only when the cells must use lactose as a fuel source. 

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