Gene Regulation in Prokaryotes (Exam point of view).pptx

1. Basic Concept of Gene Regulation

In prokaryotic cells such as Escherichia coli, genes are often organized into operons, which are clusters of genes controlled by a single promoter and transcribed as a single mRNA molecule (polycistronic mRNA). Each operon typically encodes enzymes or proteins that function in a common metabolic pathway. The expression of these genes can be induced (turned on) or repressed (turned off) depending on the cellular needs.

The process of gene expression involves two main steps:

Transcription – synthesis of mRNA from DNA by RNA polymerase.

Translation – synthesis of proteins from mRNA by ribosomes.

Gene regulation in prokaryotes primarily occurs at the transcriptional level, although control at translational and post-translational levels also exists.

2. Components Involved in Gene Regulation

Several genetic elements and proteins interact to control transcription:

Promoter: A DNA sequence where RNA polymerase binds to initiate transcription.

Operator: A regulatory sequence located adjacent to the promoter; it acts as the binding site for repressor proteins.

Regulatory genes: These encode regulatory proteins such as repressors or activators that influence transcription.

Effector molecules: Small molecules (such as inducers or co-repressors) that interact with regulatory proteins to modify their activity.

3. The Operon Model of Gene Regulation

The operon model, first proposed by François Jacob and Jacques Monod in 1961, is the cornerstone of understanding gene regulation in prokaryotes. It describes how groups of functionally related genes are coordinately regulated.

Two classical examples are the lac operon and the trp operon in E. coli.

a) The Lac Operon (Inducible System)

The lac operon controls the metabolism of lactose in E. coli. It consists of three structural genes:

lacZ – encodes β-galactosidase (breaks lactose into glucose and galactose)

lacY – encodes permease (facilitates lactose entry into the cell)

lacA – encodes transacetylase (detoxification role)

The operon also includes a promoter (P), operator (O), and a regulatory gene (lacI) located elsewhere, which encodes the lac repressor.

In absence of lactose: The lac repressor binds to the operator, blocking RNA polymerase from transcribing the structural genes. Thus, the operon is off.

In presence of lactose: Lactose is converted to allolactose, an inducer that binds to the repressor and inactivates it. The repressor cannot bind the operator, allowing RNA polymerase to transcribe the genes. The operon is on, leading to synthesis of lactose-metabolizing enzymes.

The lac operon also exhibits catabolite repression: when glucose is abundant, cyclic AMP (cAMP) levels fall, and the cAMP–CAP complex (catabolite activator protein) cannot stimulate transcription. Thus, E. coli prefers glucose over lactose, demonstrating dual control—both negative and positive regulation.

b) The Tryptophan Operon (Repressible System)

The trp operon controls the

1. Presented By Sijo A Ph.D. Research Scholar (Microbiology) School of Biosciences, MACFAST College Tiruvalla, Kerala GENE REGULATION IN PROKARYOTES

2. Gene regulation Gene regulation is the process by which a cell controls the expression of its genes to respond efficiently to environmental changes. In prokaryotes (like bacteria), gene regulation is often organized through a system called the operon , allowing coordinated control of functionally related genes.

3. Operon concept The operon model was first proposed by François Jacob and Jacques Monod in 1961 to explain how bacteria regulate gene expression. An operon is a functional unit of DNA that includes a group of structural genes under the control of a single promoter and regulated by a common operator . All genes in an operon are transcribed together into a single polycistronic mRNA , allowing simultaneous expression of multiple proteins involved in the same pathway (e.g., lactose metabolism).

4. Components of an Operon An operon typically consists of the following elements: Component Function Promoter DNA sequence where RNA polymerase binds to initiate transcription. Operator Regulatory DNA sequence where repressor or activator proteins bind to control transcription. Structural genes Genes that are transcribed together and code for proteins with related functions. Regulator gene Located outside the operon; codes for regulatory proteins (e.g., repressors or activators). Terminator Sequence that signals end of transcription.

5. Lac Operon (inducible): Controls lactose metabolism in E. coli. Trp Operon (repressible): Controls tryptophan biosynthesis . Ara Operon: Regulates arabinose metabolism . Significance of Operon-Based Regulation Enables energy-efficient gene expression . Allows bacteria to rapidly adapt to environmental changes. Coordinates expression of multiple genes under a single regulatory mechanism . EXAMPLES OF PROKARYOTIC OPERONS

6. Gene expression in prokaryotes can be regulated through: 1. Negative Regulation A repressor protein binds to the operator and blocks transcription . RNA polymerase cannot access the promoter when the repressor is bound. Can be of two types: Inducible system : Repressor is active until an inducer inactivates it (e.g., lac operon ). Repressible system : Repressor is inactive until activated by a corepressor (e.g., trp operon ). Example – Lac Operon (Negative Inducible): In the absence of lactose, the lac repressor binds to the operator and blocks transcription. When lactose is present, it acts as an inducer , binding to the repressor and inactivating it → transcription proceeds. Example – Trp Operon (Negative Repressible): When tryptophan is abundant, it acts as a corepressor , activating the trp repressor → blocks the operon and halts tryptophan synthesis. MODES OF GENE REGULATION

7. 2 . Positive Regulation An activator protein enhances transcription by helping RNA polymerase bind to the promoter. Requires an effector molecule (e.g., cyclic AMP). Example – Lac Operon (Positive Control): When glucose is low, cAMP levels rise . cAMP binds to CAP (catabolite activator protein), which then binds near the promoter and enhances RNA polymerase binding . Result: Lac genes are highly expressed only when glucose is low and lactose is present . MODES OF GENE REGULATION