The lac operon is an inducible operon that utilizes lactose as an energy source and is activated when glucose is low and lactose is present.
However, for the lac operon to be activated, two conditions must be met. First, the level of glucose must be very low or non-existent. Second, lactose must be present. Only when glucose is absent and lactose is present will the lac operon be transcribed (Figure 2).
Regulation of the lac Operon The activity of the promoter that controls the expression of the lac operon is regulated by two different proteins. One of the proteins prevents the RNA polymerase from transcribing (negative control), the other enhances the binding of RNA polymerase to the promoter (positive control).
Bacterial genes are often found in operons. Genes in an operon are transcribed as a group and have a single promoter. Each operon contains regulatory DNA sequences, which act as binding sites for regulatory proteins that promote or inhibit transcription.
How does the cell recognize that lactose needs to be metabolized? -lactose directly activates the transcription of the lac operon. -lactose stimulates a transcription factor that activates transcription of the lac operon. -lactose is converted into allolactose, which inhibits the lac repressor.
By binding repressors, they prevent the formation of active complexes between RNA polymerase and promoters. Here we show that the Escherichia coli lac operator-repressor complex also efficiently interrupts ongoing transcription.
The lac operon consists of three contiguous structural genes that are transcribed as continuous mRNA by RNA polymerase. An operator sequence located at the 5′ end serves as a binding site for a repressor protein that blocks RNA polymerase.
In their classic paper they described the regulatory mechanism of the lac operon of Escherichia coli, a system that allows the bacterium to repress the production of enzymes involved in lactose metabolism when lactose is not available.
The repressor binds to an operator that is located between the lacI gene and the beginning of the lacZ gene and prevents transcription of the structural genes of the operon [1].
An activator binds within the regulatory region of an operon, helping RNA polymerase bind to the promoter, thereby enhancing the transcription of this operon. An inducer influences transcription through interacting with a repressor or activator. The trp operon is a classic example of a repressible operon.
In bacteria, two enzymes involved in lactose metabolism have been recognized; β-galactosidase hydrolyzes the disaccharide lactose to glucose and galactose, while phospho-β-galactosidase cleaves lactose 6-phosphate to glucose and galactose 6-phosphate.
When lactose is not present, the DNA-binding protein called ► lac repressor binds to a region called the operator, which switches the lac operon off. When lactose binds to the repressor, it causes the repressor to fall off the operator, turning ► the operon on.