Bacterial Gene Expression and Regulation

Introduction

• Not every gene in a cell is expressed– Allows individual bacteria to adjust their metabolism to environmental

changes.

• 2 main ways of controlling metabolism:1. Regulate enzyme activity - catalytic activity increased or decreased due to chemical cues. Useful for immediate short-term responses.

ex. Feedback inhibition - end product acts as inhibitor of

main pathway.

2. Regulate gene expression - cellular metabolic changes switch genes “off” or “on” and increase or decrease enzyme concentrations. Slower but long-term response.

ex. Gene Induction and Gene Repression

Inducer - A substance that cause a gene’s expression to switched “on”.

Ex. utilization of lactose (Lac operon), found in catabolic pathways

Repressor - The product of a pathway thataccumulates and blocks synthesis of a required enzyme within the pathway. Thus, turning “off” the gene for the required enzyme and blocking further production of the end product.

Ex. Biosynthesis of tryptophan (trp operon) found in anabolic pathways

Repressors and Inducers

• Both are often allosteric proteins (2 binding sites) whose activities are affected by ligand binding

• Both have a function to play involving the lac operon and regulation of most other catabolic pathways.

Gene StructureOperon- A sequence of DNA that directs a

particular biosynthetic pathway. 4 Major components:

1. Regulatory gene - produces a repressor protein.2. Promoter - Sequence of DNA where the RNA

Polymerase attaches to begin transcription.3. Operator - Region of DNA that, if occupied by a

repressor protein, can block RNA Polymerase.

4. Structural Gene - DNA sequence that codes for several related enzymes that direct production of the end product.

The lac Operon

In E. coli, controls the breakdown of lactose lactose -galactosidase glucose + galactose

Has 3 structural genes:

1. lac Z - codes for -galactosidase.

2. lac Y - codes for permease (lactose transporter)

3. lac A - codes for transacetylase (role unknown)

Induction of the Lac Operon

Illustration of the method by which the lac operon is transcribed.

Glucose and lactose levels control the initiation of transcription of the lac operon in E. coli (whether the lac operon is switched "ON" or "OFF”).

* In an E. Coli cell growing in the absence of lactose, a repressor protein binds to the operator, preventing RNA polymerase from transcribing the lac operon's genes. The operon is “OFF”. * When the inducer, lactose, is added, it binds to the repressor and changes the repressor's shape so as to detach from the operator. As long as the operator remains free of the repressor, RNA polymerase can recognize the promoter and transcribe the operon's structural genes into mRNA. The operon is “ON”.

The trp Operon

In E. coli, controls the enzymes for the synthesis of the amino acid typtophan.

Repression of the trp Operon

If the level of tryptophan inside the cell is low, the tryptophan repressor protein does not bind tryptophan and thus cannot bind to its control region - the operator (green) -within the promoter (yellow). RNA polymerase can then bind to the promoter and transcribe the five genes of the tryptophan operon (left). If the level of tryptophan is high, however, the repressor protein binds tryptophan, in which state it can bind to the operator, where it blocks the binding of RNA polymerase to the promoter (right). Whenever the level of intracellular tryptophan drops, the repressor releases its tryptophan and is released from the DNA, allowing the polymerase to again transcribe the operon.

Links• http://scitec.uwichill.edu

.bb/bcs/courses/BL14D/Lac%20operon2.htm• http://www.langara.bc.ca/biology/mario/Biol2430notes/

biol2430chap13.html • http://www.bio.cmu.edu/Courses/03121/fall/26.html • http://srv2.lycoming

.edu/~newman/courses/bio22298/regulation.htm • http://www.langara.bc

.ca/biology/mario/Biol2315notes/biol2315chap10.html • http://www.accessexcellence.org/AB/GG/induction.html• http://www.accessexcellence.org/AB/GG/repressProt.html• http://w3.dwm.ks.edu.tw/bio/activelearner/13/ch13c1.html