Gene regulation in prokaryotes

The prokaryotes DNA is organised into a circular chromosome and is supercoiled in the cell cytoplasm’s nucleoid region. Proteins that are required for a particular function, or those included in the same biochemical pathway, are encoded in the blocks together and are called operons. So, for instance, all the genes that are required to rely on lactose as some energy source are coded beside each other in the lactose or, as said, lac operon.

Regulatory Molecules in Prokaryotic cells

In prokaryotic cells, three regulatory molecules can affect the operon expression. These are activators, repressors, and then inducers. 

Repressors, for example, are those proteins that help suppress a gene transcription in response to an external stimulus. On the other hand, activators are those proteins that help in increasing the gene transcription in response to an external stimulus. And then finally comes the role of inducers, which are small molecules that activate or might repress transcription based on the cell’s requirements and the substrate availability.

The Trp Operon

It is a Repressor Operon. Bacteria like E. coli require amino acids for survival. Tryptophan is a kind of amino acid that E. coli may ingest from the surroundings. Tryptophan may get synthesised utilising the five gene encode enzymes. These five genes are beside each other and are called the tryptophan operon.

If tryptophan is available in the outside environment, then E. coli need not synthesise it, and the switch that controls the activation of the gene in the operon on the top is switched off. Although, when the availability of tryptophan is low, the switch that controls the operon is turned on, and the transcription is initiated.

Also, genes are expressed, and the tryptophan gets synthesised. Five genes required for the synthesis of tryptophan are located beside each other in the top operon in E. coli. When there is plenty of tryptophan, there are two tryptophan molecules that help in binding the repressor protein and the operator sequence.

However, when it is absent, the repressor protein cannot bind to the operator, and thus, the genes get transcribed.

 Catabolite Activator Protein (CAP)

 It is an Activator Regulator, just like the top operon, is negatively regulated by tryptophan molecules protein operator sequences and act as a positive regulator to turn the genes on and thus, activate them.

 When the levels of glucose drop, cyclic AMP (cAMP) starts to accumulate in the cell, the cyclic AMP molecule is a basic molecule that is included in glucose and the energy metabolism in E. coli. When glucose levels reduce in the cell, accumulating cyclic AMP gets bound to the positive regulator that is catabolite activator protein. It is a protein that helps bind the promoters of operons as it controls the alternative sugar processing.

When cAMP gets bound to CAP, the complex gets bound to the gene promoter region that is required to use the sugar sources that may work as some alternative. A CAP binding site in these operons is placed on the upper side of the RNA polymerase that binds the site in the promoter. This helps increase the RNA polymerase binding ability to the promoter region and the transcription.

When the level of the glucose falls, E. coli can use some other sugars for meeting its fuel needs but should transcribe the new genes to do that. As the glucose supplies become limited, the level of the cAMP increases. This cAMP binds to the CAP protein, a positive regulator.

 The Lac Operon

 It is an Inducer Operon and the gene regulation of the third type in cells of prokaryotic via inducible operons. It has proteins that help bind to activate and repress the transcription based on the local environment and the cell needs. The lac operon is a common inducible operon.

For the lac operon to be transcribed, there should be a condition where the glucose is absent and lactose is present. This makes sense because the cell would be wasteful to make the proteins to process the lactose if the glucose is plentiful. If the glucose is absent, then the CAP may bind to the operator sequence to activate the transcription. However, if the lactose is not present, then the repressor is bound to the operator to prevent transcription.

If either of the requirements is met, then the transcription remains off. But only when both these conditions are met the lac operon will get transcribed. The gene expression regulation in prokaryotic cells happens at the level of transcription.

Conclusion

Tryptophan is a kind of amino acid that is essential to make proteins. However, if the tryptophan is present in abundance, it is a waste to make more, and the top receptor’s expression is repressed.