Gene Expression in Prokaryotes.

Gene expression in prokaryotes

Genetic expression is the process by which inheritable information from a gene, such as the DNA sequence, is made into a functional gene product, such as protein or RNA

Non-protein coding genes (e.g. rRNA genes, tRNA genes) are transcribed, but not translated into protein

Group of organisms lacking a cell nucleus or any other membrane-bound organelles.

Differ from the eukaryotes, which have a cell nucleus

Mode of division is binary fission Prokaryotes exhibit efficient genetic

mechanisms to respond to environmental conditions

Rod-Shaped Bacterium, Escherichia coli dividing by binary fission

Rod-Shaped Bacterium,

hemorrhagic E. coli, strain

* * * * *

Type B

Inducible gene− Regulated by inducer/activator

Constitutive gene− Not subjected to regulation

Control at the level of transcription Induction - the production of a

specific enzyme/s in response to the presence of a substrate

Repression - the cessation of production of a specific enzyme/s in response to an increased level of a substrate

All of the genes which encode the enzymes necessary for the pathway are found next to each other on the E. coli chromosome

A single mRNA carries information for multiple proteins

This type of mRNA is called a polycistronic mRNA and is totally unique to prokaryotes

An operon model is a self-regulating series of genes found on DNA that work in concert

It includes a special segment of genes that are regulators of the protein synthesis, but do not code for protein, called the promoter and operator regions

Lactose (Lac), Tryptophan (Trp), L-Arabinose (Ara)

Inducible system

Three genes part of an operon that code for three separate enzymes

Needed for the breakdown of lactose, a simple sugar

Lac IPromot

er gene

Operator

geneLac Z Lac Y Lac A

RNA polymerase

Repressortetramer

R

Translation & Transcription

Inactive repressor

Desired product

Lac IPromot

er gene

Operator

geneLac Z Lac Y Lac A

R

Translation & Transcription

RNA polymerase

No Gene

Expression

Lac IPromot

er gene

Operator

geneLac Z Lac Y Lac A

mRNA

R R

RR

Lactose absentRepressormolecules

Repressortetramer

No Gene

Expression

RNA polymerase

Lac IPromot

er gene

Operator

geneLac Z Lac Y Lac A

mRNA

R R

RR

RNA polymerase

mRNA

Thiogalactoside

transacetylase

Permease

-galactosid

ase Inducer

Inactive repressor

R

Lactose/ Isopropyl

Thiogalactoside (IPTG) present

If there occurs no glucose metabolism

Lac I Promoter gene

Operator gene Lac Z Lac Y Lac A

cAMP

Glucose pool gets depleted due to metabolism

CAP-cAMP complex formed

cAMP

RNA polymerase

mRNA

-galactosidase

Permease

Thiogalactoside transacetylase

If there occurs glucose metabolism

R

IR

I

Trp Operon ModelTrp A

Trp C

Trp B

Trp LP/O Trp

ETrp D

Tryptophan

synthetase (B

protein)

Tryptophan

synthetase (A

protein)

Anthranilate

synthetase

Anthranilate

synthetase

Glycerol-phosphat

e syntheta

se

1. L-arabinose operon model

2. Controlled by a dual positive and negative system

3. 3 structural genes: araB, araA, and araD

4. Encode the metabolic enzymes for breaking down arabinose for further metabolism via the HMP Shunt pathway