Gene Expression in Prokaryotes

Introduction

Bacteria are often dismissed as simple organisms. The procedure that acts to induce and reduce the expression of a gene is referred to as gene regulation. 

Some of these proteins are required regularly, while others are only required under specific circumstances. As a result, cells do not always express all of the genes in their genome. 

Regulation of Gene 

Gene regulation refers to the different systems that govern which genes are expressed and at what levels. However, a lot of gene regulation happens at the transcriptional level.

Bacterial regulatory molecules control whether or not a gene is translated into mRNA. These chemicals typically function by binding to specific DNA sites around the gene and assist or inhibit the transcription enzymes RNA polymerase. 

Prokaryotes Gene Regulation

Prokaryotes’ DNA is supercoiled within the nucleoid area of the cell cytoplasm and arranged into a circular chromosome. Operons are units made of linked genes that regulate genes responsible for protein synthesis. Proteins required for a certain function or engaged in the same metabolic process are encoded in operons, blocks of coding. The lactose (or lac) operon, for example, codes for all of the genes required to utilise lactose as an energy source and is transcribed into a single mRNA.

Repressors, activators, and inducers are three regulatory molecules that can alter operon expression in prokaryotes organisms. Proteins called repressors and activators are made in the cell. Repressors and activators influence gene expression by binding to particular DNA locations near their regulated genes. Repressors bind to operator regions, while activators attach to the promoter site. Activators boost gene transcription in response to an external stimulus, whereas repressors block gene transcription in reaction to an external stimulus. Small chemicals created by the cell or found in the cell’s surroundings are known as inducers. Depending on the cell’s demands and substrate availability, inducers either activate or suppress transcription.

Although eukaryotic genes lack operons, bacterial operons serve as excellent models for learning about gene regulation in general. Each operon contains DNA sequences that govern its transcription in a region known as the regulatory region. The promoter and the area around the promoter are part of the regulatory region, which transcription factors, proteins produced by regulatory genes, can bind to. Transcription factors affect RNA polymerase’s ability to bind to the promoter and advance to transcribe structural genes. 

The binding of repressors prevents RNA polymerase from transcribing structural genes. On the other hand, an activator is a transcription factor that facilitates RNA polymerase binding to the promoter of a gene in response to an external stimulus. An inducer, the third kind of regulatory molecule, is a tiny molecule that interacts with a repressor or an activator to either stimulate or inhibit transcription.

The trp Operon: A Repressor Operon

Amino acids are required for bacteria like E. coli to live. E. coli can absorb tryptophan, an amino acid found in the environment. Using enzymes produced by five genes, E. coli can also manufacture tryptophan. The tryptophan (trp) operon is encoded with five genes. If tryptophan is already available in the environment, E. coli does not need to generate it, and the switch regulating the trp operon’s gene activity is turned off. When tryptophan levels are low, the operon’s control switch is activated, transcription begins, the genes are expressed in prokaryotes, and tryptophan is produced. Here, the tryptophan itself acts as a repressor.

Between the first trp coding gene and promoter region, a DNA sequence known as the operator sequence is encoded. The code of DNA to which the repressor protein can bind is included in this operator. The tryptophan is there in the cell; 2 tryptophan molecules connect to the trp repressor, altering the form to bind to the trp operator. The tryptophan–repressor complex physically stops the RNA polymerase from joining and transcribing the downstream genes when it binds to the operator.

Because the repressor doesn’t bind to the operator when tryptophan is absent in the cell, the operon becomes active, and tryptophan is produced. As a result, the top operon is regulated negatively. In addition, the proteins that bind to the operator to quiet trp production are negative (-ve) regulators since the repressor protein actively attaches to it to keep the genes switched off.

Catabolite Activator Protein (CAP): An Activator Regulator

Similar to how tryptophan molecules negatively control the trp operon or gene expression in prokaryotes, proteins that bind to operator sequences operate as positive regulators to switch genes on and activate them. When glucose is limited, E. coli bacteria can use alternative sugar sources as a source of energy. To do this, additional genes must be produced to process the alternative genes. When glucose levels decline, the cell accumulates cyclic AMP (cAMP). In E. coli, the cAMP molecule is a signalling molecule involved in glucose and energy metabolism.

When glucose levels in the cell fall, cAMP accumulates and binds to the catabolite activator protein (CAP). This protein binds to the promoters of operons that regulate alternative sugar metabolism. When cAMP interacts with CAP, the complex binds to the promoter region of the genes involved in using alternative sugar sources. A CAP binding site is situated upstream of the RNA polymerase binding site in the promoter in these operons. This improves RNA polymerase’s capacity to bind to the promoter region and promotes gene transcription. 

The lac Operon: An Inducer Operon

Transcriptional operons, which have proteins that bind to repress or activate transcription, which depends on the local environment and the cell’s demands, are the third form of gene regulation in prokaryotes organisms. As previously indicated, when glucose concentrations are low, E. coli can use other sugars as energy sources. The cAMP–CAP protein complex works as the positive regulator for inducing transcription in this approach. One such sugar source is lactose. The lac operon includes the genes necessary for obtaining and digesting lactose from the environment.

However, two criteria must be satisfied for the lac operon to be active. First and foremost, the glucose level must be extremely low or non-existent. Lactose, on the other hand, is required. The lac operon is only transcribed when lactose is present and glucose is absent. This makes sense for the cell since creating the proteins to digest lactose would be energy inefficient if glucose was abundant or lactose was unavailable. 

• When Glucose is present and lactose is absent, the cAMP levels are low, leaving the activator CAP inactive. Hence, there’s no binding of the RNA polymerase and no transcription as well
• When both glucose and lactose are present, the CAP activator remains inactive, and the lac repressor isn’t functional as lactose, an inducer is present. Hence, basal level transcription occurs
• When glucose and lactose are absent, the CAP activator is high as cAMP is also present. Yet, the lac repressor is functional and hence prevents transcription
• When glucose is absent and lactose is present, cAMP levels are high, leading to the active binding of CAP and RNA polymerase. The lac repressor is inactive since lactose and an inducer is present. Hence, effective transcription occurs 

Conclusion

Genes in prokaryotes are organised in operons, DNA sections containing a promoter, an operator, and one or more genes that encode proteins required for a specific purpose. Lac operon regulation is a good example of bacterial gene regulation. When lactose is present, RNA polymerase attaches to the promoter, turning on the operon; the lac repressor binds to the operator; when lactose is absent, turning down the operon. The top operon demonstrates negative regulation. Attenuation, a mechanism involving RNA secondary structures, regulates the trp operon.