Question

Is the lac operon a negative or positive control system?

Answer 1

In E. coli and many other enteric bacteria, the lactose operon (lac operon) is necessary for lactose transport and metabolism. Although glucose is most bacteria's preferred carbon source, the lac operon enables for successful lactose digestion when glucose isn't available due to the activity of beta-galactosidase. Lac operon gene regulation was the first genetic regulatory mechanism to be properly described, and it has since become a leading example of prokaryotic gene regulation. For this reason, it is frequently discussed in introductory molecular and cellular biology classes.

Complete answer:
Operon: A set of genes transcribed under the control of an operator gene. More specifically, an operon is a segment of DNA containing adjacent genes including structural genes, an operator gene, and a regulatory gene. An operon is thus a functional unit of transcription and genetic regulation.
Both mechanisms are present in the lac operon. Because expression is often restricted by an active repressor (the lac repressor) that turns off transcription, it is a negative control system. The lac repressor attaches to the operator region and inhibits transcription from taking place.
This is an example of a positive control system when CAP (catabolite gene activating protein) attaches upstream of this operator region near the promoter and transcription increases. When glucose levels fall, we witness this positive control of transcription.
The lac operon's inducer–repressor control is an example of negative control, in which expression is generally suppressed. The CAP-cAMP system, on the other hand, is a positive control system since it requires the existence of an activating signal—in this case, the interaction of the CAP-cAMP complex with the CAP region—for production of the lac operon.
To complete their job, activators and repressor proteins must be able to switch between two states: one that can bind DNA targets and one that cannot. For a particular set of environmental conditions, the binding state must be appropriate. DNA binding is regulated in many activator or repressor proteins by the interaction of two distinct sites in the three-dimensional structure of the protein. The DNA-binding domain is one of them.
The allosteric region works as a switch, allowing the DNA-binding domain to be switched between functional and nonfunctional modes. Allosteric effectors are tiny compounds that interact with the allosteric site.
Lac inducers are allosteric effectors that regulate the lac operon. When allosteric effectors bind to the allosteric site, the regulatory protein undergoes a conformational change that changes the structure of the DNA-binding domain. To bind DNA, some activator or repressor proteins must first bind to their allosteric effectors. Others can only bind DNA when their allosteric effectors aren't present.

Note:-
LacZ codes for -galactosidase, which cleaves lactose into galactose and glucose; lacY codes for lac permease, a transmembrane protein required for lactose absorption; and lacA codes for a transacetylase that transfers an acetyl group. E. coli 's lac operon Lactose metabolism genes are found in E. coli. When lactose is present but glucose is not, it is expressed. The lac repressor and catabolite activator protein are two regulators that turn the operon "on" and "off" in response to lactose and glucose levels (CAP).