Control Mechanisms Control Mechanisms for Gene Expressionfor Gene Expression

Genes Gone Wild?!?!Genes Gone Wild?!?!Remember, it takes energy to do make Remember, it takes energy to do make proteins and if they are not needed at that proteins and if they are not needed at that moment, you shouldn’t waste energy moment, you shouldn’t waste energy making them.making them.Some proteins are vital to cell survival and Some proteins are vital to cell survival and are needed all the time – are needed all the time – housekeeping housekeeping genesgenes. Others are only needed in specific . Others are only needed in specific circumstances.circumstances.Gene regulationGene regulation is vital to an organism’s is vital to an organism’s survival. A cell must be able to turn survival. A cell must be able to turn specific genes on or off depending on that specific genes on or off depending on that cell’s specific requirements.cell’s specific requirements.

Eukaryotic Gene ControlEukaryotic Gene ControlGenes in eukaryotes are controlled in four Genes in eukaryotes are controlled in four manners:manners:

TranscriptionalTranscriptional – factors can control whether a – factors can control whether a gene is transcribed or not.gene is transcribed or not.Post-transcriptionalPost-transcriptional – the mRNA modifications – the mRNA modifications must be made before it can be released.must be made before it can be released.TranslationalTranslational – Factors determine how often and – Factors determine how often and frequently the mRNA is translated by ribosomes frequently the mRNA is translated by ribosomes before the cytoplasmic enzymes destroy it.before the cytoplasmic enzymes destroy it.Post-translationalPost-translational – Even after the protein is – Even after the protein is made it may not be activated right away or made it may not be activated right away or release from the cell may be delayed if release from the cell may be delayed if necessary.necessary.

Prokaryotic Gene ControlProkaryotic Gene ControlOperonsOperons are clusters of genes that operate under the control are clusters of genes that operate under the control of a of a promoterpromoter and an and an operatoroperator. They usually look after a . They usually look after a specific job in the cell that requires several enzymes in order specific job in the cell that requires several enzymes in order to get the job done.to get the job done.Operons are found mainly in prokaryotic cells but there are Operons are found mainly in prokaryotic cells but there are instances in which lower eukaryotes possess operons.instances in which lower eukaryotes possess operons.

The The promoterpromoter is a region within the DNA that controls the is a region within the DNA that controls the transcription of a gene. It usually lies just upstream of the transcription of a gene. It usually lies just upstream of the gene it regulates.gene it regulates.The The operatoroperator is an region within the DNA to which regulatory is an region within the DNA to which regulatory proteins can bind. The operator lies just next to (or overlaps) proteins can bind. The operator lies just next to (or overlaps) with the promoter.with the promoter.Regulatory proteinsRegulatory proteins bind to the operator to either proceed bind to the operator to either proceed with or stop transcription.with or stop transcription.

Parts of an OperonParts of an OperonThe The promoterpromoter is a region within the DNA that is a region within the DNA that controls the transcription of a gene. It usually lies controls the transcription of a gene. It usually lies just upstream of the gene it regulates.just upstream of the gene it regulates.The The operatoroperator is an region within the DNA to which is an region within the DNA to which regulatory proteins can bind. The operator lies just regulatory proteins can bind. The operator lies just next to (or overlaps) with the promoter.next to (or overlaps) with the promoter.

The operator has a direct impact on the promoters ability The operator has a direct impact on the promoters ability to allow transcription to proceed (or not). to allow transcription to proceed (or not).

Regulatory proteinsRegulatory proteins bind to the operator to bind to the operator to either proceed with or stop transcription.either proceed with or stop transcription.

Examples of regulatory proteins are repressors and Examples of regulatory proteins are repressors and corepressors – we will see what they do shortly!corepressors – we will see what they do shortly!

The lac OperonThe lac OperonThe lac operon looks after the breakdown The lac operon looks after the breakdown of lactose in bacterial cells. The enzyme of lactose in bacterial cells. The enzyme that breaks down the lactose is called that breaks down the lactose is called β-β-galactosidasegalactosidase..As with any enzyme, it takes energy to As with any enzyme, it takes energy to make, so it wouldn’t make much sense for make, so it wouldn’t make much sense for the bacterial cell to make this enzyme the bacterial cell to make this enzyme unless lactose was present.unless lactose was present.The cell uses a negative control system to The cell uses a negative control system to control the production of the enzyme. Only control the production of the enzyme. Only a change within the cell triggers the a change within the cell triggers the operon’s function.operon’s function.

The lac OperonThe lac OperonThe successful breakdown of lactose depends on three genes – lac The successful breakdown of lactose depends on three genes – lac Z, lac Y and lac A. These genes are located on the same stretch of Z, lac Y and lac A. These genes are located on the same stretch of DNA along with the operon’s promoter and operator regions, which DNA along with the operon’s promoter and operator regions, which overlap just a bit.overlap just a bit.

When lactose is not present in the cell, a When lactose is not present in the cell, a repressor proteinrepressor protein called called the LacI protein binds to the operator and covers part of the the LacI protein binds to the operator and covers part of the promoter – they do overlap. This stops the RNA polymerase from promoter – they do overlap. This stops the RNA polymerase from binding from the promoter and transcribing their codes. The gene binding from the promoter and transcribing their codes. The gene products are not made and the cell saves energy.products are not made and the cell saves energy.

When the bacterial cell takes in some lactose, the lactose acts as an When the bacterial cell takes in some lactose, the lactose acts as an inducerinducer and binds to the LacI repressor and changes its shape so it and binds to the LacI repressor and changes its shape so it can no longer bind to the operator and promoter. With the repressor can no longer bind to the operator and promoter. With the repressor no longer acting as a roadblock, the RNA polymerase makes the no longer acting as a roadblock, the RNA polymerase makes the copies of the genes and the enzymes are made to breakdown the copies of the genes and the enzymes are made to breakdown the lactose.lactose.Once the lactose is all used up (including the one acting as an Once the lactose is all used up (including the one acting as an inducer), the LacI repressor goes back and binds to the operator and inducer), the LacI repressor goes back and binds to the operator and covers part of the promoter and the genes are basically shut down.covers part of the promoter and the genes are basically shut down.

The lac OperonThe lac Operon

The trp OperonThe trp OperonThe The trp operontrp operon controls the production of tryptophan – an amino controls the production of tryptophan – an amino acid – and it consists of five genes, an operator and a promoter.acid – and it consists of five genes, an operator and a promoter.It differs from the lac operon in that it shuts down when high It differs from the lac operon in that it shuts down when high levels of tryptophan are present. There is no need to make if it the levels of tryptophan are present. There is no need to make if it the cell already has it. This makes tryptophan the effector.cell already has it. This makes tryptophan the effector.

When tryptophan enters the cell is binds to a repressor molecule When tryptophan enters the cell is binds to a repressor molecule to form the trp repressor-tryptophan complex. This complex then to form the trp repressor-tryptophan complex. This complex then binds to the operator and does not allow the RNA polymerase to binds to the operator and does not allow the RNA polymerase to transcribe the genes of the operon.transcribe the genes of the operon.Tryptophan is termed a Tryptophan is termed a corepressorcorepressor because it is needed to bind because it is needed to bind to the repressor in order to activate it.to the repressor in order to activate it.When the tryptophan taken in by the cell is all used up – including When the tryptophan taken in by the cell is all used up – including the one acting as a corepressor – the cell must begin to make its the one acting as a corepressor – the cell must begin to make its own tryptophan again. Once the corepressor is gone, the trp own tryptophan again. Once the corepressor is gone, the trp repressor falls off the operator and the genes needed to make repressor falls off the operator and the genes needed to make tryptophan can begin to function again.tryptophan can begin to function again.

The trp OperonThe trp Operon

That’s All I Got…That’s All I Got…