Operons and Gene Regulation

In this tutorial, you will learn about operons, including their structure and function, as well as the specific mechanisms of the Lac, arabinose, and tryptophan operons. You will explore the differences between inducible and repressible operons, understand the regulation of gene expression through positive and negative controls, and examine the roles of housekeeping and regulatory genes in cellular functions.

Contents:

1. Introduction to Operons
2. Lac Operon
3. Lac Operon: Gene Regulation in Lactose Metabolism
4. Arabinose Operon
5. Components of the Arabinose Operon
6. Tryptophan Operon
7. Tryptophan Operon Components and Functions
8. Comparing the Lac Operon and Trp Operon
9. Regulation of Gene Expression
10. Positive and Negative Gene Regulation
11. Housekeeping vs. Regulatory Genes
12. Repressible and Inducible Genes

Introduction to Operons

Operons are fundamental units of genetic regulation in prokaryotes, consisting of a cluster of genes under the control of a single promoter and regulatory elements. They allow for coordinated expression of genes that are functionally related, facilitating the efficient management of metabolic processes in response to environmental changes.

Lac Operon

The Lac operon is another classic example of an inducible operon. This operon is responsible for the breakdown of lactose into glucose and galactose.

Mechanism of action:

• The Lac operon encodes three structural genes that are necessary to acquired and processed the disaccharide into two monosaccharides galactose and glucose.
• For the expression of Lac operon lactose must be present. In absence of disaccharide lactose, the repressor is bound to the operator region, preventing RNA Polymerase from transcribing three different structural genes.
• In presence of lactose, it is converted into allolactose and allolactose acts as an inducer binding to repressor. This binding of inducer to repressor prevents binding of repressor to operator region. This removal of repressor molecule allows RNA Polymerase begin transcription of Lac Z, Lac Y and Lac A structural genes.

Lac Operon: Gene Regulation in Lactose Metabolism

Lac operon unit consists of three structural genes, inducer site, promoter site etc. Lac means lactose and this operon is helpful incase of lactose metabolism.

• This figure depicts about the Lac operon unit which is an inducible operon. Inducible operon is the type of operon which induces the expression of genes.
• X is operator site, and it is regulatory in nature. When Lac repressor binds to the operator site, it prevents the transcription reaction to proceed. When it is not bounded by lac repressor, the transcription reaction proceeds.
• Three structural genes are present. Lac Z codes for Beta-galactosidase, Lac A codes for transacetylase and Lac Y codes for permease.

Arabinose Operon

Arabinose operon is an example of inducible operon. This operon is unique with respect to other inducible operons. Here the same regulatory protein is capable of exerting negative as well as positive control.

Mechanism of action:

• Arabinose metabolism is mainly governed by enzymatic products of three different structural genes, Ara D, Ara A and Ara B whereas the transcription is controlled by AraC (Regulatory protein) encoded by AraC gene. This AraC protein interacts with two regulatory regions called araI and araO2. These sites can bound individually or coordinately by Ara C protein.
• I region bears that designation because when this I region is bound by Ara C protein, the system is induced. For this binding to occur both Arabinose and cAMP must be present. CAP binding site is present in promoter region that modulates catabolite repression in presence of glucose.
• In absence of both arabinose and cAMP, the Ara C protein binds coordinately to both I site and O2 site. When both I and O2 are bound by Ara C, a confirmational change occurs in DNA to form a tight loop and the structural genes are repressed.

Components of the Arabinose Operon

Arabinose is an example of inducible operon which helps is arabinose metabolism.

• This diagram illustrates about the working unit of arabinose operon. X is Ara C which is a regulatory gene. Transcription is controlled by Ara C and is encoded by Ara C gene.
• Y is CAP site. Cap site along with ara I makes up the inducer site. CAP site modulates catabolite repression in presence of glucose. Catabolite repression occurs in presence of glucose in envieronment.
• There are three different structural genes present in arabinose operon. Ara B, Ara A and Ara D are the structural genes that encodes for three different metabolic enzymes.

Tryptophan Operon

Tryptophan operon is a type of repressible operon which is regulated by tryptophan (trp) repressor. This trp repressor when binds to tryptophan, it blocks operon’s expression. This mechanism of tryptophan biosynthesis is also regulated by a mechanism called attenuation.

• Five genes are present in trp operon i.e., trp E, trp D, trp C, trp B and trp A. Promoter region and operator region are also present.
• Trp repressor prevents binding of RNA polymerase to promoter region by binding to operator region. This blocks RNA polymerase activity of transcription.
• The trp operon is switched off when tryptophan is present in environment. So, there is no need of synthesizing extra tryptophan.
• The trp operon is switched on when tryptophan is absent in environment. So, there is a need of synthesizing extra tryptophan.
• Trp repressor play the most important role of a sensor in trp operon by switching it off and switching it on. It happens according to the situation.

Tryptophan Operon Components and Functions

Tryptophan operon is an example of repressible operon. The repressible operon represses when tryptophan levels are high and expresses when tryptophan levels are low.

• This figure gives information about the working unit of tryptophan operon which was found in E. coli bacteria. But this operon is present in many organisms.
• X is operator region of tryptophan operon. It is a binding site for tryptophan (trp)repressor. Trp repressor binds to this operator region and hinders the RNA polymerase and blocks the transcription.
• Tryptophan operon consists of five different structural genes i.e., trp A, trp B, trp C, trp D and trp E. These structural genes codes for different metabolic enzymes that are helpful in trp operon mechanism.

Comparing the Lac Operon and Trp Operon

The following table compares the difference between Lac Operon and Trp Operon.

Regulation of Gene Expression

Gene expression in prokaryotes and eukaryotes is tightly regulated at multiple stages to ensure proper cellular function and adaptation to environmental changes. Several key processes are involved in maintaining the cellular concentration of proteins:

• Transcription: The synthesis of primary RNA transcripts from DNA.
• RNA processing: Post-transcriptional modifications such as 5′ capping and 3′ polyadenylation.
• mRNA degradation: Messenger RNA molecules are degraded by pathways like the deadenylation-dependent pathway.
• Translation: The conversion of mRNA into a polypeptide chain.
• Post-translational modification: Modifications that lead to the biologically active form of the protein.
• Protein targeting and transport: The distribution of proteins to their functional locations within or outside the cell.
• Protein degradation: Controlled breakdown of proteins when they are no longer needed.

Positive and Negative Gene Regulation

Gene regulation can occur through positive and negative control mechanisms.

• Positive regulation: In this mode, activator proteins enhance the binding of RNA polymerase to the promoter, facilitating transcription. For example, the CAP-cAMP complex in the lac operon serves as a positive regulator in the absence of glucose.
• Negative regulation: Here, repressor proteins bind to the operator region, blocking RNA polymerase and preventing transcription. This occurs in both the trp operon (in the presence of tryptophan) and the lac operon (in the absence of lactose).

Housekeeping vs. Regulatory Genes

• Housekeeping genes: These genes are expressed at consistent levels in almost all cell types because they are essential for basic cellular functions like energy metabolism and DNA replication.
• Regulatory genes: In contrast, these genes regulate the expression of other genes depending on environmental signals. Regulatory genes can be either inducible or repressible, depending on whether their expression is increased (induction) or decreased (repression) in response to specific signals.

Repressible and Inducible Genes

Inducible genes are the genes which are responsible for inducing nature of operon. Unlike inducible genes, repressible genes are responsible for repressing nature of operon.

• Gene products whose concentration decreases in response to molecular signal are called repressible genes. The process involved is called repression.
• These repressible genes are expressive genes until they are repressed by any compound. They play a major role in tryptophan (trp) operon.
• Gene products whose concentration increases in response to molecular signal are called inducible genes. The process involved is called induction.
• Inducible genes are responsive in nature to changes around their environment and adapt to conditions. This aids in support in case of inducible operon like Lac operon and arabinose operon.