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Biochemistry

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Purines, Pyrimidines and Nucleic Acid Metabolism

Question
21 out of 59
 

CAP in lac operon is (AIIMS Nov 2010)



A Positive regulator

B Negative regulator

C Constitutive expression

D Non constitutive expression

Ans. A Positive regulator
The lactose operon

a. The lactose operon codes for three proteins involved in the catabolism of the disaccharide, lactose: The lacZ gene codes for β-galactosidase, which hydrolyzes lactose to galactose and glucose; the lacY gene, which codes for a permease that facilitates the movement of lactose into the cell; and the lacA gene that codes for thiogalactoside transacetylase whose exact physiologic function is unknown.

b. All of these proteins are produced when lactose is available to the cell but glucose is not.

[Note: Bacteria use glucose as a fuel in preference to any other sugar.] The regulatory portion of the operon is upstream of the three structural genes, and consists of the promoter (P) region where RNA polymerase binds, and two additional sites, the operator (O) site and the CAP site, where regulatory proteins bind. The lacZ, lacY, and lacA genes are expressed only when the O site is empty, and the CAP site is bound by a complex of cyclic adenosine monophosphate (cAMP, see p. 94) and the catabolite gene activator protein or CAP (sometimes called the cAMP

c. regulatory protein or CRP, A regulatory gene, the lacI gene, codes for the repressor protein (a trans-acting factor) that binds to the operator site. [Note: The lacI gene has its own promoter.]

d. When glucose is the only sugar available: In this case, the lac operon is repressed (turned off). Repression is mediated by the repressor protein binding via a helix-turn-helix motif to the operator site, which is downstream of the promoter region. Binding of the repressor interferes with the progress of RNA polymerase, and blocks transcription of the structural genes. This is an example of negative regulation.

e. When only lactose is available: In this case, the lac operon is induced (expressed or turned on). A small amount of lactose is converted to an isomer, allolactose. This compound is an inducer that binds to the repressor protein, changing its conformation so that it can no longer bind to the operator. In the absence of glucose, adenylyl cyclase is active, and sufficient quantities of cAMP are made and bind to the CAP protein.

f. The cAMP–CAP complex binds to the CAP-binding site, causing RNA polymerase to more efficiently initiate transcription at the promoter site. This is an example of positive regulation. The transcript is a single polycistronic mRNA molecule that contains three sets of start and stop codons.

g. Translation of the mRNA produces the three proteins that allow lactose to be used for energy production by the cell. [Note: In contrast to the inducible lacZ, lacY, and lacA genes, whose expression is regulated, the lacI gene is constitutive. Its gene product, the repressor protein, is active unless the inducer is present.]

h. When both glucose and lactose are available: In this case, transcription of the lac operon is negligible, even if lactose is present at a high concentration. Adenylyl cyclase is deactivated in the presence of glucose—a process known as catabolite repression—so no cAMP–CAP complex forms and the CAP-binding site remains empty. RNA polymerase is, therefore, unable to effectively initiate transcription, even though the repressor may not be bound to the operator region. Consequently, the three structural genes are not expressed

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

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