Enzymes operon regulation

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. 

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. 

FIGURE 28-19 The trp operon. This operon is regulated by two mechanisms when tryptophan levels are high, (1) the repressor (upper left) binds to its operator and (2) transcription of trp mRNA is attenuated (see Fig. 28-21). The biosynthesis of tryptophan by the enzymes encoded in the trp operon is diagrammed at the bottom... 

Regulation of histidine synthesis. In all, ten different genes code for the enzymes of histidine biosynthesis in Salmonella typhimurium. They are clustered as the histidine operon, a consecutive series of genes which are transcribed into messenger RNA as a unit.250 251 The gene symbols His A, HisB, etc., are indicated in Fig. 25-13, and their positions on the E. coli gene map are indicated in Fig. 26-4. The gene HisB codes for a complex protein with two different enzymatic activities as shown in Fig. 25-13. 

Enzymes That Catalyze Amino Acid Biosynthesis Are Regulated at the Level of Transcription Initiation The trp Operon Is Also Regulated after the Initiation Point for Transcription Genes for Ribosomes Are Coordinately Regulated Control of rRNA and tRNA Synthesis by the rel Gene... 

The trp operon contains a cluster of five structural genes associated with tryptophan biosynthesis. Initiation of transcription of the trp operon is regulated by a repressor protein that functions similarly to the lac repressor. The main difference is that the trp repressor action is subject to control by the small-molecule effector, tryptophan. When tryptophan binds the repressor, the repressor binds to the trp operator. Thus, the effect of the small-molecule effector here is opposite to its effect on the lac operon. When tryptophan is present, there is no need for the enzymes that synthesize tryptophan. 

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