<< Chapter < Page Chapter >> Page >

Prokaryotic promoters

A promoter    is a DNA sequence onto which the transcription machinery binds and initiates transcription. In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all the time, some of the time, or infrequently. Although promoters vary among prokaryotic genomes, a few elements are conserved. At the -10 and -35 regions upstream of the initiation site, there are two promoter consensus    sequences, or regions that are similar across all promoters and across various bacterial species ( [link] ). The -10 consensus sequence, called the -10 region, is TATAAT. The -35 sequence, TTGACA, is recognized and bound by σ . Once this interaction is made, the subunits of the core enzyme bind to the site. The A–T-rich -10 region facilitates unwinding of the DNA template, and several phosphodiester bonds are made. The transcription initiation phase ends with the production of abortive transcripts, which are polymers of approximately 10 nucleotides that are made and released.

Illustration shows the σ subunit of RNA polymerase bound to two consensus sequences that are 10 and 35 bases upstream of the transcription start site. RNA polymerase is bound to σ.
The σ subunit of prokaryotic RNA polymerase recognizes consensus sequences found in the promoter region upstream of the transcription start sight. The σ subunit dissociates from the polymerase after transcription has been initiated.

View this MolecularMovies animation to see the first part of transcription and the base sequence repetition of the TATA box.

Elongation and termination in prokaryotes

The transcription elongation phase begins with the release of the σ subunit from the polymerase. The dissociation of σ allows the core enzyme to proceed along the DNA template, synthesizing mRNA in the 5' to 3' direction at a rate of approximately 40 nucleotides per second. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it ( [link] ). The base pairing between DNA and RNA is not stable enough to maintain the stability of the mRNA synthesis components. Instead, the RNA polymerase acts as a stable linker between the DNA template and the nascent RNA strands to ensure that elongation is not interrupted prematurely.

Illustration shows RNA synthesis by RNA polymerase. The RNA strand is synthesized in the 5' to 3' direction.
During elongation, the prokaryotic RNA polymerase tracks along the DNA template, synthesizes mRNA in the 5' to 3' direction, and unwinds and rewinds the DNA as it is read.

Prokaryotic termination signals

Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA. Depending on the gene being transcribed, there are two kinds of termination signals. One is protein-based and the other is RNA-based. Rho-dependent termination    is controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Near the end of the gene, the polymerase encounters a run of G nucleotides on the DNA template and it stalls. As a result, the rho protein collides with the polymerase. The interaction with rho releases the mRNA from the transcription bubble.

Rho-independent termination is controlled by specific sequences in the DNA template strand. As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C–G nucleotides. The mRNA folds back on itself, and the complementary C–G nucleotides bind together. The result is a stable hairpin    that causes the polymerase to stall as soon as it begins to transcribe a region rich in A–T nucleotides. The complementary U–A region of the mRNA transcript forms only a weak interaction with the template DNA. This, coupled with the stalled polymerase, induces enough instability for the core enzyme to break away and liberate the new mRNA transcript.

Upon termination, the process of transcription is complete. By the time termination occurs, the prokaryotic transcript would already have been used to begin synthesis of numerous copies of the encoded protein because these processes can occur concurrently. The unification of transcription, translation, and even mRNA degradation is possible because all of these processes occur in the same 5' to 3' direction, and because there is no membranous compartmentalization in the prokaryotic cell ( [link] ). In contrast, the presence of a nucleus in eukaryotic cells precludes simultaneous transcription and translation.

Illustration shows multiple mRNAs transcribed off one gene. Ribosomes attach to the mRNA before transcription is complete and begin to make protein.
Multiple polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate the mRNA transcripts into polypeptides. In this way, a specific protein can rapidly reach a high concentration in the bacterial cell.

Visit this BioStudio animation to see the process of prokaryotic transcription.

Section summary

In prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template comprising two consensus sequences that recruit RNA polymerase. The prokaryotic polymerase consists of a core enzyme of four protein subunits and a σ protein that assists only with initiation. Elongation synthesizes mRNA in the 5' to 3' direction at a rate of 40 nucleotides per second. Termination liberates the mRNA and occurs either by rho protein interaction or by the formation of an mRNA hairpin.

Questions & Answers

what is plant
Odion Reply
what are organisms
Godfred Reply
the diagram below show how the blood of a human embryo flows close to the mothers blood in the placenta . which substances are represent at x in higher concentrations than at y
Joyce Reply
what is ecology
Odion Reply
what is cell
what is cell
What is a cell
Awal Reply
What are tissues
A cell is the smallest unit of living organisms.
Tissues are group of similar cells performing a particular function.
how to pronounce the word cyanobacteria
siva Reply
Cyano then bacteria. look that "cyano" is just like "ciyano".
explain the term transpiration pull
Tank Reply
water is mainly " pulled" through the plant and that the driving force for this process is transpiration from the leaves .This is also referred to as the cohesion- tension -transpiration pull model transport.
what Is corona
My brother go and ask GOOGLE😷😷😷
Google it for the right answer. Corona is a virus which causes Novel Covid-19 disease that has been declared by WHO as a global or world pandemic.
Why is it called Covid-19?
jjwhat is. condinative system
condinative system is the system that harmonize all the. different. part. of. the. body and make. them function as a. specific unit
Describe the process of protein sythesis?
Kizito Reply
Ribosomes are responsible for protein synthesis in animals.
What is a ploidy level
Francis Reply
Ploidy refers to the number of chromosomes. We have 23 pairs of chromosomes in somatic cells. Sex cells are haploid thus 23 chromosomes vs. 46 Chromosomes.
Evolution is evolvement according to one's environment. Let's use humidity as an example. A person from a very cold environment would not be used to hot humid weather. But over time their body would slowly, slowly adapt.
Give me (3) types of biodiversity
Hi guys 🇧🇪
hello boy
what do you guys understand about evolution?
Kiana Reply
it's a process or analayzing some program
but am not sure if some know plz let share
is a study of
unfolding or unrolling hence the process of development of growth
is the periodic change of the structural features of an organism
I think
process of developing
hi guiz
hello guys
Evolution ~ Is the gradual change of development over time in which a new species are formed
Evolution is the sum total of adaptive changes that has existed before.
why are diagrams not available
Evans Reply
How can nucleotide molecules of DNA be constructed?construct a six molecules of DNA?
Julia Reply
You take a Deoxyribose sugar. The Deoxyribose sugar has a hydrogen on the second carbon vs. an OH group. The nitrogenous base binds to the #1 Carbon and a phosphate binds to the 5'. When the 2nd NBase bonds, it does so on the 3' Carbon. Thus you have 5' to 3' directional growth.
what is the explanation of local biomes in Nigeria
It would be savanna, tropical grassland.
describe the breathing mechanism in the body fish.
Jackson Reply
what is genetics.
the study of hereditary
state the importance of biodiversity of organisms in an area?
Chris Reply
Let's compare The Brazilian Rain Forest to Antarctica. There many, many types of flora and fona in The Rain Forest. So many that we are still discovering them vs. the variety in Antarctica.
it boosts the ecosystem where each species has an important tole

Get the best Biology course in your pocket!

Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

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