<< Chapter < Page Chapter >> Page >
A long strand of DNA shows RNA polymerase bound to a promoter upstream of gene A. Many various shapes are attached to the promoter and RNA polymerase. These are transcription factors and mediator proteins. These also bind to distant regions of DNA called distal control elements and activators. This requires DNA to bend (so the distant regions can reach the promoter). This bending is due to a DNA bending protein.
In eukaryotes, an enhancer is a DNA sequence that promotes transcription. Each enhancer is made up of short DNA sequences called distal control elements. Activators bound to the distal control elements interact with mediator proteins and transcription factors. Two different genes may have the same promoter but different distal control elements, enabling differential gene expression.

Dna-level control

In eukaryotes, the DNA molecules or associated histones can be chemically modified in such a way as to influence transcription; this is called epigenetic regulation . Methylation of certain cytosine nucleotides in DNA in response to environmental factors has been shown to influence use of such DNA for transcription, with DNA methylation commonly correlating to lowered levels of gene expression. Additionally, in response to environmental factors, histone proteins for packaging DNA can also be chemically modified in multiple ways, including acetylation and deacetylation, influencing the packaging state of DNA and thus affecting the availability of loosely wound DNA for transcription. These chemical modifications can sometimes be maintained through multiple rounds of cell division, making at least some of these epigenetic changes heritable.

  • What stops or allows transcription to proceed when attenuation is operating?
  • What determines the state of a riboswitch?
  • Describe the function of an enhancer.
  • Describe two mechanisms of epigenetic regulation in eukaryotes.


Although Mark survived his bout with necrotizing fasciitis, he would now have to undergo a skin-grafting surgery, followed by long-term physical therapy. Based on the amount of muscle mass he lost, it is unlikely that his leg will return to full strength, but his physical therapist is optimistic that he will regain some use of his leg.

Laboratory testing revealed the causative agent of Mark’s infection was a strain of group A streptococcus (Group A strep). As required by law, Mark’s case was reported to the state health department and ultimately to the Centers for Disease Control and Prevention (CDC). At the CDC, the strain of group A strep isolated from Mark was analyzed more thoroughly for methicillin resistance.

Methicillin resistance is genetically encoded and is becoming more common in group A strep through horizontal gene transfer. In necrotizing fasciitis, blood flow to the infected area is typically limited because of the action of various genetically encoded bacterial toxins. This is why there is typically little to no bleeding as a result of the incision test. Unfortunately, these bacterial toxins limit the effectiveness of intravenous antibiotics in clearing infection from the skin and underlying tissue, meaning that antibiotic resistance alone does not explain the ineffectiveness of Mark’s treatment. Nevertheless, intravenous antibiotic therapy was warranted to help minimize the possible outcome of sepsis, which is a common outcome of necrotizing fasciitis. Through genomic analysis by the CDC of the strain isolated from Mark, several of the important virulence genes were shown to be encoded on prophages, indicating that transduction is important in the horizontal gene transfer of these genes from one bacterial cell to another.

Go back to the previous Clinical Focus box.

Key concepts and summary

  • Gene expression is a tightly regulated process.
  • Gene expression in prokaryotes is largely regulated at the point of transcription. Gene expression in eukaryotes is additionally regulated post-transcriptionally.
  • Prokaryotic structural genes of related function are often organized into operons , all controlled by transcription from a single promoter. The regulatory region of an operon includes the promoter itself and the region surrounding the promoter to which transcription factors can bind to influence transcription.
  • Although some operons are constitutively expressed , most are subject to regulation through the use of transcription factors (repressors and activators). A repressor binds to an operator , a DNA sequence within the regulatory region between the RNA polymerase binding site in the promoter and first structural gene, thereby physically blocking transcription of these operons. An activator binds within the regulatory region of an operon, helping RNA polymerase bind to the promoter, thereby enhancing the transcription of this operon. An inducer influences transcription through interacting with a repressor or activator.
  • The trp operon is a classic example of a repressible operon . When tryptophan accumulates, tryptophan binds to a repressor, which then binds to the operator, preventing further transcription.
  • The lac operon is a classic example an inducible operon . When lactose is present in the cell, it is converted to allolactose. Allolactose acts as an inducer, binding to the repressor and preventing the repressor from binding to the operator. This allows transcription of the structural genes.
  • The lac operon is also subject to activation. When glucose levels are depleted, some cellular ATP is converted into cAMP, which binds to the catabolite activator protein (CAP) . The cAMP-CAP complex activates transcription of the lac operon. When glucose levels are high, its presence prevents transcription of the lac operon and other operons by catabolite repression .
  • Small intracellular molecules called alarmones are made in response to various environmental stresses, allowing bacteria to control the transcription of a group of operons, called a regulon.
  • Bacteria have the ability to change which σ factor of RNA polymerase they use in response to environmental conditions to quickly and globally change which regulons are transcribed.
  • Prokaryotes have regulatory mechanisms, including attenuation and the use of riboswitches , to simultaneously control the completion of transcription and translation from that transcript. These mechanisms work through the formation of stem loops in the 5’ end of an mRNA molecule currently being synthesized.
  • There are additional points of regulation of gene expression in prokaryotes and eukaryotes. In eukaryotes, epigenetic regulation by chemical modification of DNA or histones, and regulation of RNA processing are two methods.

Fill in the blank

The DNA sequence, to which repressors may bind, that lies between the promoter and the first structural gene is called the ________.


Got questions? Get instant answers now!

The prevention of expression of operons encoding substrate use pathways for substrates other than glucose when glucose is present is called _______.

catabolite repression

Got questions? Get instant answers now!

Short answer

What are two ways that bacteria can influence the transcription of multiple different operons simultaneously in response to a particular environmental condition?

Got questions? Get instant answers now!

Questions & Answers

what is cell?
Anita Reply
the cell is the functional and structural unit of the body
yes ur right
who add me
this is not the correct answer
Alanna Reply
why not
what is the correct answer?
where is question
ask you question
What are mitochondria
what is mitochondria
who is the father of microbiology
Antonie Van Luweenhoek
Mitochondria is the powerhouse of the Cell. And is the cell organelle which helps cell in generation of energy by Cellular Respiration
what is a vacual
Star Reply
What is microbiology
Nji Reply
it is the study of microscopic organisms and basically their relation to the environment
what is elements
Erneet Reply
an Element is a substance that can not be further decomposed by ordinary chemical means.
can i get notes of biochemistry
Sneha Reply
yep why not
what is microbiology?
Gayanjali Reply
what is malaria
bios- life/living things micro- small things that cannot be seen with the naked eye In other words the study of living things that cannot be seen with the naked eye but with the help of a microscope
logos- science
it is 10 to the power minus 6 biology which studies life forms on those scales including metabolism , physiology, nutrition requirements
what are prokarytes
A prokaryotic is a unicellular organanism that lacks a membrane bound nucleus ,mitochondria or other any membrane bound organelle. .......
in prokaryotic nucleus is not developed and cell organelles are absent
how is one who is feeling blockage in the chest with mucus be diagnosed
Essy Reply
be talking sepitol every morning
is that all?
What is prions
Sangeeta Reply
is tuberculosis a viral infection?
hildar Reply
No. It is a Bacterial Infection.
what are zn staining
Evelyn Reply
full meaning zeil neelson staining
By using zn stain we diagnose / examine acid fast bacilli .
explain hw bacteria grows
Kanyago Reply
Hw does bacteria grow?
Binary fission
three successive phase comes in bacteria growth i.e lag phase/adaptation phase ,log phase/exponential phase ,death phase when it is in fermentation process .
introduction to virology
Dongyile Reply
is the study of virus
study of virus which can be seen by using electron microscope i.e below 1micron in size . It requires a host cell to multiply own number to proliferate itself .
what are some five benefits of studying microbiology
Joyce Reply
please I'm waiting for answers
microbio is very important! 1.) microbes thought to have evolved into our cells 2.) they keep us healthy 3.) they can cause disease and illness 4.) good for bioremediation 5.) show evolution in real time(observable) 6.) make the earth a liveable place (make more o2 than plants)
What fields are interesting to study when you are doing your postgraduate degree

Get the best Microbiology course in your pocket!

Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Microbiology' conversation and receive update notifications?