Gene Regulation In Prokaryotes Flashcards
How is genetic expression regulated in prokaryotes ?
Bacteria regulate gene expression in response to environmental conditions
- E. Coli has about 4000 different possible polypeptide chains encoded for the genome
- Some proteins have only 5-10 molecules in the cell
- Others, such as ribosomal proteins and proteins in the glycolytic pathway can have as many as 100,000 copies in the cell
- Some proteins have very few copies (basal levels) which can be increased dramatically when required.
- There are regulatory mechanism to control the expression of genetic information
What are inducible genes?
Enzymes which are synthesized on,y when required based upon the chemical makeup of the environment are called inducible genes which are inducible expressed
- turning off inducible genes refferred to as repression or repressible - The presence of a specific molecule inhibits gene expression
What are constitutive genes?
Enzymes which are produced continuously, regardless of the chemical makeup of the environment are called constitutive enzymes from constitutive genes which are constitutively expressed.
- Typically low level, constant expression - Some gene products of constitutive genes regulate the expression of inducible genes
Explain inducible gene expression
Either positive or negative
A gene (or group of genes) can have both positive and negative control as seen with the metabolism of lactose in E. Coli.
Negative control- genetic expression occurs unless it is shut off by some sort of a regulator molecule
Positive control- genetic expression occurs only if a regulator molecule directly stimulates RNA production
What are the 3 types of molecules affecting expression in prokaryotic genes?
Prokaryotic cells use the following:
-Repressors
- Activators
- Inducers
Repressor sand activators are proteins produced in the cell which bind to DNA regions close to the genes that they will control
Inducers are small molecules either produced in the cell or taken up in to the cell, which can either activate or repress transcription, depending on needs of the cell at a particular time
What are regulatory regions?
Regulatory regions are DNA sequences on the same strand near the polycistronic cluster (promoter and operator)
What are activators and repressors?
The protein molecules that bind the regulatory regions upstream of the structural genes
- Can positively or negatively regulate the transcription of the structural genes
- Negatively by turning off transcription
- Positively by turning on transcription
How do structural genes play a role in prokaryotic DNA regulation?
- Structural genes encoding for enzymes are often organized in clusters (polycistronic) depending on function or pathway
- Transcription of these genes is under the control of a single regulatory DNA region located upstream of the cluster that they control.
What is a promoter?
- RNA polymerase recognizes the promoter sequence (pribnow box) and will bind to it
- RNA polymerase will scan along the DNA till it finds the transcription initiation (start) site for the structural gene
- RNA polymerase will begin transcribing the RNA of the structural gene
What is an operator?
- the operator is a sequence of DNA downstream of the promoter which is recognized by a repressor protein
- If the repressor protein in bound, RNA polymerase can not synthesize RNA
How is lactose metabolism in E. coli is regulated by an Inducible system?
- Francis Jacob and Jacques Monod were studying metabolism in E. coli
- Glucose is the preferred food source for bacteria
- Switched food source to lactose and studied what happened to gene expression
- 3 genes were expressed to protein, this is also called coordinate regulation
What is the preferred sugar source of bacteria?
Glucose
How is glucose metabolized in bacteria?
- Glucose is metabolized most efficiently by bacteria, first step is conversion to glucose 6-P
- If there is no glucose in the environment but there is lactose, an enzyme called B-galactosidase will cleave lactose to glucose and galactose
- The glucose portion is metabolized easily to glucose 6-P
- The galactose must go through many steps to be converted to glucose 6-P
Metabolism of lactose is not as efficient as pure glucose
What happens if glucose becomes depleted in the bacterial environment and lactose is present, what happens?
- The enzymes responsible for the metabolism of lactose increases rapidly from a few molecules to thousands per cell
- The enzymes responsible for lactose metabolism are inducible
- Lactose is the inducer (after it converted to allolactose)
What are Operons?
In bacteria, genes that encode enzymes with related functions tend to be organized in clusters
-These gene clusters are often under the coordinated control of a single regulatory region, entire region is called an Operon
How can lactose be metabolized for energy?
Lactose can be metabolized for energy by bacteria
-First step is the cleavage of lactose to galactose and glucose
- The enzyme is B-galactosidase (gene is LacZ)
- A minor activity of B-galactosidase is the conversion of lactose to allolactose
-Glucose can be metabolized directly but galactose must be converted glucose 6-phosphate before it is metabolized by the bacteria for energy
Describe the transcription of Lac operon structural genes
-RNA polymerase will bind to the promoter and transcribe the structural genes in a single unit (polycistronic RNA) including : lacZ, lacA, and lacY
The single RNA is translated into three gene products
Entire gene cluster functions to provide rapid response to the presence of absence of lactose
Describe the basics of Lac Operon
There are 3 enzymes required to metabolize lactose
- B-galactose, permease and transacetylase
These 3 genes are called structural genes and they are clustered together in a tip to tail fashion (polycistronic)
-lacZ, LacY and lacA
- One regulatory region, promoter and operator
- One repressor gene, lacl which encodes for a repressor protein
