CONTROL OF GENE EXPRESSIONThe development of an

organism must involve the switching on and off of genes

in an orderly manner. This is not fully understood in multi-cellular organisms, but

has been researched in bacteria….

Genetic Switches in Bacteria

• Francois Jacob and Jacques Monod worked with E.coli bacteria in 1960

• E.coli grown in a medium containing lactose produces the enzyme lactase (β-galactosidase) which hydrolyses lactose to glucose and galactose

• lactose + lactase glucose + galactose

• Glucose and galactose can then be used to supply energy to the bacterium

• If there is no lactose in the medium, the bacteria produce no lactase

• However, once lactose is added, production of the enzyme (lactase) begins

• By timing enzyme production to suit the circumstances, energy is saved

• In order for lactose to be used it must enter the cell first

• Lactose permease is a membrane protein that enables lactose to enter the cell

• Lactose permease production is also induced by lactose

Jacob and Monod set out to investigate the process of enzyme induction

• This involved looking at mutant bacteria which produced lactase constantly (even in the absence of lactose)

• The lactase produced by the mutants was indistinguishable from the ‘normal’ lactase

• This indicated that the gene specifying the structure of the enzyme was quite different from the gene controlling its rate of production

• i.e. there were 2 genes, one coding for the structure and one coding for the production of the protein (the mutation was in the one coding for the production)

Jacob and Monod distinguished between 2 types of gene

• STRUCTURAL GENES– Specify the amino acid sequence of

the polypeptide– The genes coding for lactase and

lactose permease are structural

• REGULATOR GENES– Which control the level of activity of

the structural genes

• The mutants produced lactase and lactose permease constantly, which showed they were controlled by the same switch

• After mapping the genes, they found that they were next to each other on the bacterial chromosome

• These 2 genes and their ‘switch’ were acting as one unit – an operon

• This is the proposed mechanism

• http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/lacoperon.html

The lac operon

• The lac operon is made up of regulator genes, structural genes and a promoter and operator region

• The regulator gene codes for a repressor protein

• When lactose is absent, the repressor protein binds to the operator region and prevents RNA polymerase from transcribing the gene

• When lactose is present it binds to the repressor protein and changes its shape

• The inactivated repressor protein is then unable to bind to the operator region of the operon

• Since the inactive repressor protein is unable to bind to the operator region, RNA polymerase (the enzyme responsible for the transcription of genes) is now able to bind to the promoter region of the operon

• RNA polymerase is now able to transcribe the three enzyme genes (Z, Y, and A) into mRNA.

• With the transcription of these genes, the three enzymes needed for the bacterium to utilize the sugar lactose are now synthesized.

– The Z gene codes for lactase, an enzyme that breaks down lactose into glucose and galactose.

– The Y gene codes for permease, an enzyme which transports lactose into the bacterium.

• The transcription of the structural genes allows the breakdown of lactose to glucose and galactose

• As the lactose is broken down, releasing energy, the repressor protein is released and can bind to the operator region again

• Transcription of the genes is once again stopped

In summary:

• When lactose is not present, the genes in the lac operon are not expressed.

• A repressor, which is always present in the cell, binds to the lac operon and prevents transcription by blocking the passage of RNA polymerase.

• However, when lactose is present, it binds to the repressor and changes its shape, such that the repressor can no longer bind to the operon.

• In this case, RNA polymerase proceeds along the operon and transcribes all 3 genes.

• The product of these genes metabolises lactose, releasing the repressor and allowing it to once again block RNA polymerase.

• The lac operon is an inducible system, meaning that the system is turned off until an inducer – lactose – arrives on the scene

Control of gene expression in eukaryotes

• This is much more complicated due to the numbers and arrangement of genes within the genome

• Several genes may be responsible for one characteristic, and while they may be clustered together, the mechanisms that control them may be located on different chromosomes

• Eukaryotic genomes are much larger

• They contain repetitive sequences of DNA and introns (non-coding regions which RNA splices out during transcription)

• Each gene in eukaryotes has its own set of regulatory sequences that control its expression

Transcription control is based on 2 fundamental control sequences• PROMOTERS

– Found before the binding site for RNA polymerase, and allows it to bind to its recognition site

• ENHANCERS– Can be located at a great distance

from the gene, and can be before or after the promoter region

Transcription factors

• In addition to promoters and enhancers, there are a large number of proteins called TRANSCRIPTION FACTORS involved in control– Some bind to enhancers, some to the

promoter to increase transcription

• Most models of control of transcription in eukaryotes involve the formation of a complex between enhancers, transcription factors, the promoter and RNA polymerase resulting in the formation of a loop of DNA that initiates transcription

Eukaryotic gene expression

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html#