It includes structural genes generally encoding enzymes , regulatory genes encoding, e. The type of control is defined by the response of the operon when no regulatory protein is present.
In the case of negative control , the genes in the operon are expressed unless they are switched off by a repressor protein. Thus the operon will be turned on constitutively the genes will be expressed when the repressor in inactivated.
In the case of positive control , the genes are expressed only when an active regulator protein, e. Thus the operon will be turned off when the positive regulatory protein is absent or inactivated.
Table 4. Positive vs. Catabolic pathways catalyze the breakdown of nutrients the substrate for the pathway to generate energy, or more precisely ATP, the energy currency of the cell. In the absence of the substrate , there is no reason for the catabolic enzymes to be present, and the operon encoding them is repressed. In the presence of the substrate , when the enzymes are needed, the operon is induced or de-repressed. These monosaccharides are broken down to lactate principally via glycolysis, producing ATP , and from lactate to CO2 via the citric acid cycle , producing NADH, which feeds into the electron-transport chain to produce more ATP oxidative phosphorylation.
This can provide the energy for the bacterial cell to live. However, the initial enzymes lactose permease and b-galactosidase are only needed, and only expressed, in the presence of lactose and in the absence of glucose. In the presence of the substrate lactose, the operon in turned on, and in its absence, the operon is turned off. Anabolic, or biosynthetic, pathways use energy in the form of ATP and reducing equivalents in the form of NAD P H to catalyze the synthesis of cellular components the product from simpler materials, e.
If the cell has plenty of the product already in the presence of the product , the the enzymes catalyzing its synthesis are not needed, and the operon encoding them is repressed. In the absence of the product , when the cell needs to make more, the biosynthetic operon is induced. When the cellular concentration of Trp or Trp-tRNAtrp is high, the operon is not expressed, but when the levels are low, the operon is expressed.
Inducible operons are turned on in reponse to a metabolite a small molecule undergoing metabolism that regulates the operon. Repressible operons are switched off in reponse to a small regulatory molecule.
Note that in this usage, the terms are defined by the reponse to a small molecule. Although lac is an inducible operon, we will see conditions under which it is repressed or induced via derepression. The lac operon is under both negative and positive control. The mechanisms for these will be considered separately. In negative control, the lacZYA genes are switched off by repressor when the inducer is absent signalling an absence of lactose. When the repressor tetramer is bound to o , lacZYA is not transcribed and hence not expressed.
When inducer is present signalling the presence of lactose , it binds the repressor protein, thereby altering its conformation, decreasing its affinity for o , the operator. The dissociation of the repressor-inducer complex allows lacZYA to be transcribed and therefore expressed.
The natural inducer or antirepressor , is allolactose , an analog of lactose. It is made as a metabolic by-product of the reaction catalyzed by b-galactosidase. A gratuitous inducer will induce the operon but not be metabolized by the encoded enzymes; hence the induction is maintained for a longer time. When a small amount of lactose is present the lac repressor will bind it causing dissociation from the DNA operator thus freeing the operon for gene expression. Substrates that cause repressors to dissociate from their operators are called inducers and the genes that are regulated by such repressors are called inducible genes.
Positive Control of the lac Operon. Although lactose can induce the expression of lac operon, the level of expression is very low. The reason for this is that the lac operon is subject to catabolite repression or the reduced expression of genes brought on by growth in the presence of glucose.
Glucose is very easily metabolized so is the preferred fuel source over lactose, hence it makes sense to prevent expression of lac operon when glucose is present. The strength of a promoter is determined by its ability to bind RNA polymerase and to form an open complex. The promoter for the lac operon is weak and consequently the lac operon is poorly transcribed upon induction. There is a binding site, upstream from the promoter, for a protein called the catabolite activator protein CAP.
In the presence of glucose circulating cAMP levels are very low and consequently the initiation of transcription from the lac operon is very low. The Arabinose Operon. Arabinose is a five-carbon sugar that can serve as an energy and carbon source for E. Arabinose must first be converted into ribulosephosphate before it can be metabolized. The arabinose operon has three genes, araB , araA and araD that encode for three enzymes to carry out this conversion.
A fourth gene, araC , which has its own promoter, encodes a regulatory factor called the C protein. The regulatory sites of the ara operon include four sites that bind the C protein and one CAP binding site. The other two C protein binding sites called araI 1 and araI 2 are located between the CAP binding site and the promoter.
Negative Control of the araC Operon. In the absence of arabinose, dimers of the C protein bind to araO 2 , araO 1 and araI 1. The C proteins bound to araO 2 and araI 1 associate with one another causing the DNA between them to form a loop effectively blocking transcription of the operon.
Positive Control of the araC Operon. The C protein binds arabinose and undergoes a conformational change that enables it to also bind the araO 2 and araI 2 sites. This results in the generation of a different DNA loop that is formed by the interaction of C proteins bound to the araO 1 and araO 2 sites. The formation of this loop stimulates transcription of the araC gene resulting in additional C protein synthesis, thus the C protein autoregulates its own synthesis.
C protein bound at the araI 1 and araI 2 sites interacts with the bound CAP enabling RNA polymerase to initiate transcription from the ara operon promoter. The Tryptophan Operon. Amino acid synthesis consumes a lot of energy, so to avoid wasting energy the operons that encode for amino acid synthesis are tightly regulated. The trp operon consists of five genes, trpE , trpD , trpC , trpB and trpA , that encode for the enzymes required for the synthesis of tryptophan. The trp operon is regulated by two mechanisms, negative corepression and attenuation.
Most of the operons involved in amino acid synthesis are regulated by these two mechanisms. Negative Corepression. The trp operon is negatively controlled by the trp repressor, a product of the trpR gene. The trp repressor binds to the operator and blocks transcription of the operon. However, in order to bind to the operator the repressor must first bind to Trp hence tryptophan is a corepressor.
In the absence of Trp the trp repressor dissociates and transcription of the trp operon is initiated. Attenuation regulates the termination of transcription as a function of tryptophan concentration. At low levels of trp full length mRNA is made, at high levels transcription of the trp operon is prematurely halted. Attenuation works by coupling transcription to translation. Prokaryotic mRNA does not require processing and since prokaryotes have no nucleus translation of mRNA can start before transcription is complete.
Consequently regulation of gene expression via attenuation is unique to prokaryotes. Attenuation is mediated by the formation of one of two possible stem-loop structures in a 5' segment of the trp operon in the mRNA. If tryptophan concentrations are low then translation of the leader peptide is slow and transcription of the trp operon outpaces translation.
This results in the formation of a nonterminating stem-loop structure between regions 2 and 3 in the 5' segment of the mRNA. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Consequences of Gene Regulation.
Gene Responses to Environment. Regulation of Transcription. Transcription Factors. From DNA to Protein. Organization of Chromatin. Topic rooms within Gene Expression and Regulation Close. No topic rooms are there. Or Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast.
Simply Science. Green Screen. Green Science. Bio 2. The Success Code. Why Science Matters. The Beyond. Plant ChemCast. Postcards from the Universe. Brain Metrics. Mind Read. Eyes on Environment. Accumulating Glitches. Saltwater Science. Microbe Matters. You have authorized LearnCasting of your reading list in Scitable. Do you want to LearnCast this session? This article has been posted to your Facebook page via Scitable LearnCast. Change LearnCast Settings.
When histones have acetyl groups added to them by enzymes called histone acetyl transferases HATs , the acetyl groups physically obstruct the nucleosomes from packing too densely and help to recruit other enzymes that further open the chromatin structure. Conversely, when the acetyl groups are removed by histone deacetylases HDACs , the chromatin assumes a condensed formation that prevents transcription factors from being able to access the DNA.
In the image below, you can clearly see how much more compact and inaccessible the nm fiber is top compared to the beads-on-a-string formation bottom. Chromatin plays a fundamental role in positive and negative gene regulation, because transcriptional activators and RNA polymerase cannot physically access the DNA regulatory elements when chromatin is in a compact form. Each of these processing steps is also subject to regulation, and the mRNA will be degraded if any of them are not properly completed.
The export of mRNAs from the nucleus to the cytoplasm is also regulated, as is stability of the properly processed mRNA in the cytoplasm. Finally, eukaryotic genes often have different splice variants, where different exons can be included in different mRNAs that are transcribed from the same gene.
Here you can see a cartoon of a gene with color-coded exons, and two different mRNA molecules transcribed from this gene.
The different mRNAs encode for different proteins because they contain different exons. This process is called alternative splicing and we will discuss it more here. Often different types of cells in different tissues express different splice variants of the same gene, such that there is a heart-specific transcript and a kidney-specific transcript of a particular gene.
In general, eukaryotic gene regulation is more complex than prokaryotic gene regulation. The upstream regulatory regions of eukaryotic genes have binding sites for multiple transcription factors, both positive regulators and negative regulators, that work in combination to determine the level of transcription. Some transcription factor binding sites, called enhancers and silencers, work at quite a distance, thousands of base pairs away from the promoter.
Activators are examples of positive regulation and repressors are examples of negative regulation. Eukaryotic transcription initiation, from biology. If you understand the similarities and differences in eukaryotic and prokaryotic gene regulation, then you know which of the following process are exclusive to eukaryotes, which are exclusive to prokaryotes, which occur in both, and how each is accomplished:.
The lac operon is a good model gene for understanding gene regulation. You should use the information below to make sure you can apply all of the details of gene regulation described above to a specific gene model.
0コメント