MCAT Content / Control Of Gene Expression In Prokaryotes / Positive Control In Bacteria

Positive control in bacteria

Topic: Control Of Gene Expression In Prokaryotes

Positive gene control in the bacteria takes place with the help of an operon that can increase the transcriptional activity if key enzymes. The catabolite activator protein helps in the positive control.

There are proteins that bind to the operator sequences that act as a positive regulator to turn genes on and activate them. For example, when glucose is scarce, E. coli bacteria can turn to other sugar sources for fuel. To do this, new genes to process these alternate genes must be transcribed. This type of process can be seen in the lac operon which is turned on in the presence of lactose and absence of glucose.

When glucose levels drop, cyclic AMP (cAMP) begins to accumulate in the cell. The cAMP molecule is a signaling molecule that is involved in glucose and energy metabolism in E. coli. When glucose levels decline in the cell, accumulating cAMP binds to the positive regulator catabolite activator protein (CAP), a protein that binds to the promoters of operons that control the processing of alternative sugars, such as the lac operon. The CAP assists in production in the absence of glucose.

CAP is a transcriptional activator and works with two cAMP to open up the DNA molecule, allowing RNA polymerase to bind and transcribe the genes involved in lactose catabolism. When cAMP binds to CAP, the complex binds to the promoter region of the genes that are needed to use the alternate sugar sources. In these operons, a CAP-binding site is located upstream of the RNA-polymerase-binding site in the promoter. This increases the binding ability of RNA polymerase to the promoter region and the transcription of the genes. As cAMP-CAP is required for transcription of the lac operon, this requirement reflects the greater simplicity with which glucose may be metabolized in comparison to lactose.

Whilst bacteria have positive controls to increase the binding of RNA Pol and cAMP and CAP promote transcription of operons unlike eukaryotic cells the mRNA produce does not undergo any post-transcriptional modification so all of the information transcribed from the bacterial DNA is used. There are no introns or exons and no splicing takes place.


Practice Questions


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MCAT Official Prep (AAMC)

Official Guide B/B Section Passage 3 Question 13

Sample Test B/B Section Passage 3 Question 14


Key Points

• Catabolite activator protein (CAP) must bind to cAMP to activate transcription of the lac operon by RNA polymerase.

• CAP is a transcriptional activator with a ligand-binding domain at the N-terminus and a DNA -binding domain at the C-terminus.

• cAMP molecules bind to CAP and function as allosteric effectors by increasing CAP’s affinity to DNA.

• Bacterial DNA does not undergo any post-transcriptional modification and all DNA that is converted to mRNA is used in protein-coding. There are no introns or exons.

Key Terms

RNA polymerase: a DNA-dependent RNA polymerase, an enzyme, that produces RNA

Operon: a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together

Promoter: the section of DNA that controls the initiation of RNA transcription

Lac operon: The lac operon is an operon that encodes proteins that allow the bacteria to use lactose as an energy source

Lactose: a large sugar molecule that is made up of two smaller sugar molecules, glucose and galactose

cAMP: a molecule is a signaling molecule that is involved in glucose and energy metabolism

Catabolite activator protein (CAP): a DNA binding protein involved with the transcription of several genes, including those that code for enzymes involved in the metabolism of certain sugars

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