Control Mechanisms for Gene Expression

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

Eukaryotic Gene Control Genes in eukaryotes are controlled in four manners: Transcriptional – factors can control whether a gene is transcribed or not. Post-transcriptional – the mRNA modifications must be made before it can be released. Translational – Factors determine how often and frequently the mRNA is translated by ribosomes before the cytoplasmic enzymes destroy it. Post-translational – Even after the protein is made it may not be activated right away or release from the cell may be delayed if necessary.

Prokaryotic Gene Control Operons are clusters of genes that operate under the control of a promoter and an operator. They usually look after a specific job in the cell that requires several enzymes in order to get the job done. Operons are found mainly in prokaryotic cells but there are instances in which lower eukaryotes possess operons. The promoter is a region within the DNA that controls the transcription of a gene. It usually lies just upstream of the gene it regulates. The operator is an region within the DNA to which regulatory proteins can bind. The operator lies just next to (or overlaps) with the promoter. Regulatory proteins bind to the operator to either proceed with or stop transcription.

Parts of an Operon The promoter is a region within the DNA that controls the transcription of a gene. It usually lies just upstream of the gene it regulates. The operator is an region within the DNA to which regulatory proteins can bind. The operator lies just next to (or overlaps) with the promoter. The operator has a direct impact on the promoters ability to allow transcription to proceed (or not). The operator has a direct impact on the promoters ability to allow transcription to proceed (or not). Regulatory proteins bind to the operator to either proceed with or stop transcription. Examples of regulatory proteins are repressors and corepressors – we will see what they do shortly! Examples of regulatory proteins are repressors and corepressors – we will see what they do shortly!

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

The lac Operon 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 DNA along with the operon’s promoter and operator regions, which overlap just a bit. When lactose is not present in the cell, a repressor protein called the LacI protein binds to the operator and covers part of the promoter – they do overlap. This stops the RNA polymerase from binding from the promoter and transcribing their codes. The gene products are not made and the cell saves energy. When the bacterial cell takes in some lactose, the lactose acts as an inducer and binds to the LacI repressor and changes its shape so it can no longer bind to the operator and promoter. With the repressor no longer acting as a roadblock, the RNA polymerase makes the copies of the genes and the enzymes are made to breakdown the lactose. 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 covers part of the promoter and the genes are basically shut down.

The lac Operon

The trp Operon The trp operon controls the production of tryptophan – an amino 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 levels of tryptophan are present. There is no need to make if it the cell already has it. This makes tryptophan the effector. When tryptophan enters the cell is binds to a repressor molecule to form the trp repressor-tryptophan complex. This complex then binds to the operator and does not allow the RNA polymerase to transcribe the genes of the operon. Tryptophan is termed a corepressor because it is needed to bind to the repressor in order to activate it. 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 own tryptophan again. Once the corepressor is gone, the trp repressor falls off the operator and the genes needed to make tryptophan can begin to function again.

The trp Operon

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