In bacteria and palaeontology, structural proteins with related functions (such as genes encoding enzymes that catalyse many steps in a single biochemical pathway) are usually coded together within a block within a block called an operon. And one individual promoter. This results in the formation of polycistronic transcripts. Since all of these structural genes are needed at the same time or not at all, promoters simultaneously regulate the transcriptional regulation of these structural genes. In the prokaryotes, structural genes with related functions are often organised together on the genome and transcribed together under the control of a single promoter. The operon regulatory region contains both promoters and operators. When the repressor binds to the operator, the structural gene is not transcribed. Alternatively, activators can bind to regulatory regions and enhance transcription.
Gene Regulation and Operon Concept
An operon’s expression can be controlled through repression or induction (or even a single gene). An operon’s expression can be controlled through repression or induction (or even a single gene). When a tiny metabolite binds to a regulatory repressor or inducer protein in a cell, the protein undergoes an allosteric shift, allowing it to bind to or unbind from a regulatory DNA sequence. Such regulation can be seen in the lac and trp operons. In the Lac operon, gene repression and induction are both examples of gene regulation. The Trp (tryptophan) operon is governed by gene repression. Changes in intracellular metabolite levels in both operons reflect the metabolic status of the cell and cause changes in gene transcription. We’ll investigate how both operons are controlled.
LAC Operon in Prokaryotes
Researchers François Jacob (1920–2013) and Jacques Monod at the Pasteur Institute in Paris were the first to demonstrate that bacterial genes are organised into operons through their research on the lac operon of E. coli, which they carried out in the 1950s. They discovered that in E. coli, all of the structural genes encoding enzymes required for the utilisation of lactose as an energy source are clustered together in the lactose (or lac) operon and are controlled by a single promoter, the lac promoter, which they named the lac promoter. In 1965, they were awarded the Nobel Prize in Physiology or Medicine for their research.
Operons in prokaryotic genes are excellent models for learning about gene regulation in general, despite the fact that eukaryotic genes are not organised into operons. In eukaryotes, there are some gene clusters that function in a manner similar to operons. As a result, many of the principles can be applied to eukaryotic systems, and they help us to better understand how changes in gene expression in eukaryotes can lead to pathological changes like cancer.
Each operon contains DNA sequences that have the ability to influence its own transcription; these sequences are found in a region known as the regulatory region. In the regulatory region, the promoter and the region surrounding the promoter are both present, and they are both capable of interacting with transcription factors, which are proteins encoded by regulatory genes. Transcription factors have an effect on the binding of RNA polymerase to the promoter and the progression of the enzyme through the promoter to transcribe structural genes. Repressors are transcription factors that inhibit the transcription of a gene in response to an external stimulus by binding to a DNA sequence within the regulatory region known as the operator. The operator is located between the RNA polymerase binding site of the promoter and the transcriptional start at the site of the first structural gene and is located between the promoter and the transcriptional start at the site of the first structural gene. The physical binding of a repressor prevents RNA polymerase from transcribing the structural genes.
There are operons in prokaryotes whose gene products are required rather regularly and whose expression is hence unregulated. Constitutively expressed operons are those that are continually transcribed and translated to provide the cell with consistent intermediate levels of the protein products. These genes encode enzymes involved in cellular housekeeping processes, such as DNA replication, repair, and transcription, as well as enzymes involved in core metabolism.
Regulation of Gene Expression in Prokaryotes
Gene expression is a very complex and regulated process that begins with DNA being transcribed into RNA and then translated into protein. All somatic cells in the body generally contain the same DNA. Some exceptions are red blood cells, which are mature and do not contain DNA, and immune system cells that rearrange DNA while producing antibodies. However, in general, the genes that determine the presence or absence of green eyes and brown hair and the rate of metabolism of food are the same in the cells of the eyes and liver, but the functions of these organs are mostly different.
And the DNA sequence, each cell does not turn on, or express, the same set of genes. All the cell type needs a different set of proteins to perform its function. Therefore, only the small subset of proteins is expressed in the cell that constitutes its proteome. For a protein to be expressed, the DNA must be transcribed into RNA and the RNA must be translated into protein. In the given cell type, not all genes encoded in the DNA are transcribed into RNA or translated into protein because the specific cells in our human body have specific functions. Specialised proteins that make up the eye (iris, lens, and cornea) are only expressed in the eye, whereas the specialised proteins in the human heart (pacemaker cells, heart muscle, and valves) are only expressed in the heart. At any given time period, only a subset of all genes encoded by our DNA are expressed and translated into proteins. The expression of some genes is a highly-regulated process with many levels and stages of control. This complexity ensures the proper expression in the proper cell at the proper time period.
Gene Expression Regulation in Eukaryotes
One method is by methylation and acetylation of chromatin.
- DNA acetylation (the attachment of acetyl groups onto certain sequences) on chromatin will loosen the chromatin (facilitating transition from activable euchromatin and condensed heterochromatin), thus increasing transcription and expression
- DNA methylation which attaches methyl groups onto sequences tightens the chromatin and thus decreases the chances the DNA will be transcribed
Another method of the gene expression regulation are post-transcriptional modifications which occur after a mRNA transcript has been transcribed from the DNA sequence.
- Alternative splicing may remove certain introns and thus change the exon sequence. This may mean the different genes are expressed from the same transcript
- MiRNAs (microRNAs) may tag a RNA sequence in the transcript for destruction by enzymes, hence preventing it from being expressed as a protein
Other modifications like the poly-A tail may make the transcript less prone to degradation, thus increasing in the gene expression
Conclusion
In prokaryotic organisms, transcriptional regulation regulates gene expression. Repressive control, activator control, and inducible control are the three methods for controlling an operon’s transcription. The trp operon is an example of repressive regulation, which uses proteins linked to the operator sequence to physically impede RNA polymerase binding and transcription activation. When tryptophan isn’t required, the repressor is attached to the operator, and transcription is turned off. When CAP is bound, activator control, as demonstrated by CAP’s function, increases RNA polymerase’s ability to bind to the promoter. The binding of cAMP to CAP in this scenario is caused by low glucose levels. The promoter is then bound by CAP, allowing RNA polymerase to bind to the promoter.