REGULATION OF GENE EXPRESSION & OPERON CONCEPT

What is an Operon?

* In prokaryotes, the genes coding for proteins involved in a particular metabolic pathway are often sequentially arranged together on the chromosome along with a single promoter or regulatory region. This entire cluster is termed as an operon, for eg, the lac operon (coding proteins for metabolism of lactose) or trp operon (coding proteins needed for the synthesis of tryptophan).

* An operon is regulated by the single promoter or regulatory region. All the genes of an Operon are transcribed together as a unit. The transcription product of operon is a single polycistronic messenger RNA (mRNA). Thus all the genes of an operon are controlled together, that is, they all are turned on or off as a unit.

When both glucose and lactose are available:

* In this case, transcription of the lac operon does not take place, even if lactose is present at a high concentration.

 * Presence of glucose deactivates Adenylyl cyclase—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.

* (Lac Operon regulation is very beautifully depicted in a figure of Lippincott biochemistry; Fig. 32.4 of 4/e and Fig. 30.11 of 3/e)

The CAP-cAMP regulator is acting as a positive regulator because its presence is required for gene expression.

It functions in turning on the transcription of the structural genes in the operon.

Catabolite activator protein, CAP is an activator required for high level transcription of the lac operon. CAP protein exerts positive control and lac repressor exerts negative control. 

A i.e. Positive regulator

C i.e. Inducer without glucose

C i.e. Southern Blot and Immunohistochemistry

Immunohisto chemistry detects antigens (eg proteins) in abnormal (eg cancerous) or normal cells of a tissue by antibodies binding specifically to that antigen (so immune means antigen-antibody binding and histo means in tissue). It is used to find distribution & localization of biological marker in various tissues.

 Lactose operone or Lac operon

The lac operon is a region of DNA in the genome of E. coli that contains following genetic elements ‑

i) Three structural genes :- These code for 3 proteins that are involved in catabolism of lactose. These genes are ‘Z’ gene (codes for P-galactosidase), ‘Y’ gene (codes for galactoside permease), and ‘A’ gene (codes for thiogalactoside transacetylase).

ii) Regulatory gene (lac i) It produces repressor protein.

iii) A promotor site (P) :- It is the binding site for RNA polymerase. It contains two specific regions ‑

a) CAP site (Catabolite activator protein binding site).

b) RNA polymerase binding site

iv) An operator site (0) :- Repressor binds to this site and blocks transcription.

3 Structural genes are expressed only when ‘0’ site is empty (repressor is not bound) and the CAP site is bound by a complex of cAMP and CAP (catabolite gene activator protein).

Ans. is ‘b’ i.e., Cistrone

• Gene :- It is the smallest functional unit of genome. Gene is the unit of DNA that carries the information for synthesis of specific protein.
• Cistron:- It is smallest unit of genetic expression. It is the basic genetic unit coding for one polypeptide chain, i.e., a subunit of a complete protein molecule. It is regarded as one cistron-one subunit concept. Prokaryotic mRNA is polycistronic, i.e. each mRNA has several coding region. Eukaryotic mRNA is monocistronic, i.e. each mRNA has single coding region.
• Smallest functional unit of genome → gene, codes for a specific protein.
• Smallest unit of genetic expression → cistron, (codes for the subunit of protein, i.e. one polypeptide chain).