) is a large family of RNA molecule
s that convey genetic information
to the ribosome
, where they specify the amino acid
sequence of the protein
products of gene expression
. Following transcription
of primary transcript
mRNA (known as pre-mRNA
) by RNA polymerase
, processed, mature mRNA is translated
into a polymer of amino acids: a protein, as summarized in the central dogma of molecular biology
As in DNA, mRNA genetic information is in the sequence of nucleotides
, which are arranged into codons
consisting of three base pairs each. Each codon encodes for a specific amino acid
, except the stop codon
s, which terminate protein synthesis
. This process of translation of codons into amino acids requires two other types of RNA: Transfer RNA
(tRNA), that mediates recognition of the codon and provides the corresponding amino acid, and ribosomal RNA
(rRNA), that is the central component of the ribosome's protein-manufacturing machinery.
The existence of mRNA was first suggested by Jacques Monod
and François Jacob
, and subsequently discovered by Jacob, Sydney Brenner
and Matthew Meselson
at the California Institute of Technology
It should not be confused with mitochondrial DNA
Synthesis, processing and function
The brief existence of an mRNA molecule begins with transcription, and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, edited, and transported prior to translation. Eukaryotic
mRNA molecules often require extensive processing and transport, while prokaryotic
mRNA molecules do not. A molecule of eukaryotic mRNA and the proteins surrounding it are together called a messenger RNP
Transcription is when RNA is made from DNA. During transcription, RNA polymerase
makes a copy of a gene from the DNA to mRNA as needed. This process is similar in eukaryotes and prokaryotes. One notable difference, however, is that eukaryotic RNA polymerase associates with mRNA-processing enzymes during transcription so that processing can proceed quickly after the start of transcription. The short-lived, unprocessed or partially processed product is termed precursor mRNA
, or pre-mRNA
; once completely processed, it is termed mature mRNA
Eukaryotic pre-mRNA processing
Processing of mRNA differs greatly among eukaryote
, and archea
. Non-eukaryotic mRNA is, in essence, mature upon transcription and requires no processing, except in rare cases. Eukaryotic pre-mRNA, however, requires extensive processing.
5' cap addition
A 5' cap
(also termed an RNA cap, an RNA 7-methylguanosine
cap, or an RNA m7G cap) is a modified guanine nucleotide that has been added to the "front" or 5' end
of a eukaryotic messenger RNA shortly after the start of transcription. The 5' cap consists of a terminal 7-methylguanosine residue that is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. Its presence is critical for recognition by the ribosome
and protection from RNase
Cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. Shortly after the start of transcription, the 5' end of the mRNA being synthesized is bound by a cap-synthesizing complex associated with RNA polymerase
. This enzymatic
s the chemical reactions that are required for mRNA capping. Synthesis proceeds as a multi-step biochemical
In some instances, an mRNA will be edited
, changing the nucleotide composition of that mRNA. An example in humans is the apolipoprotein B
mRNA, which is edited in some tissues, but not others. The editing creates an early stop codon, which, upon translation, produces a shorter protein.
Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms most messenger RNA (mRNA) molecules are polyadenylated at the 3' end, but recent studies have shown that short stretches of uridine (oligouridylation) are also common.Choi et al. RNA. 2012. 18: 394-401 The poly(A) tail
and the protein bound to it aid in protecting mRNA from degradation by exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. mRNA can also be polyadenylated in prokaryotic organisms, where poly(A) tails act to facilitate, rather than impede, exonucleolytic degradation.
Polyadenylation occurs during and/or immediately after transcription of DNA into RNA. After transcription has been terminated, the mRNA chain is cleaved through the action of an endonuclease complex associated with RNA polymerase. After the mRNA has been cleaved, around 250 adenosine residues are added to the free 3' end at the cleavage site. This reaction is catalyzed by polyadenylate polymerase. Just as in alternative splicing, there can be more than one polyadenylation variant of an mRNA.
Polyadenylation site mutations also occur. The primary RNA transcript of a gene is cleaved at the poly-A addition site, and 100-200 A’s are
added to the 3’ end of the RNA. If this site is altered, an abnormally long and unstable mRNA construct will be formed.
Another difference between eukaryotes and prokaryotes is mRNA transport. Because eukaryotic transcription and translation is compartmentally separated, eukaryotic mRNAs must be exported from the nucleus
to the cytoplasm
—a process that may be regulated by different signaling pathways. Mature mRNAs are recognized by their processed modifications and then exported through the nuclear pore
by binding to the cap-binding proteins CBP20 and CBP80, as well as the transcription/export complex (TREX). Multiple mRNA export pathways have been identified in eukaryotes.
In spatially complex cells, some mRNAs are transported to particular subcellar destinations. In mature neuron
s, certain mRNA are transported from the soma
s. One site of mRNA translation is at polyribosomes selectively localized beneath synapses. The mRNA for Arc/Arg3.1 is induced by synaptic activity and localizes selectively near active synapses based on signals generated by NMDA receptors. Other mRNAs also move into dendrites in response to external stimuli, such as β-actin mRNA. Upon export from the nucleus, actin mRNA associates with ZBP1
and the 40S subunit. The complex is bound by a motor protein and is transported to the target location (neurite extension) along the cytoskeleton. Eventually ZBP1 is phosphorylated by Src in order for translation to be initiated.Spatial regulation of bold beta-actin translation by Src-dependent phosphorylation of ZBP1 Nature04115
. In developing neurons, mRNAs are also transported into growing axons and especially growth cones. Many mRNAs are marked with so-called "zip codes," which target their transport to a specific location.
Because prokaryotic mRNA does not need to be processed or transported, translation by the ribosome
can begin immediately after the end of transcription. Therefore, it can be said that prokaryotic translation is coupled
to transcription and occurs co-transcriptionally
Eukaryotic mRNA that has been processed and transported to the cytoplasm (i.e., mature mRNA) can then be translated by the ribosome. Translation may occur at ribosomes
free-floating in the cytoplasm, or directed to the endoplasmic reticulum
by the signal recognition particle
. Therefore, unlike in prokaryotes, eukaryotic translation is not
directly coupled to transcription.somalia
, 5' UTR
, coding region
, 3' UTR
, and poly(A) tail.]]
Coding regions are composed of codons
, which are decoded and translated (in eukaryotes usually into one and in prokaryotes usually into several) into proteins by the ribosome. Coding regions begin with the start codon
and end with a stop codon
. In general, the start codon is an AUG triplet and the stop codon is UAA, UAG, or UGA. The coding regions tend to be stabilised by internal base pairs, this impedes degradation. In addition to being protein-coding, portions of coding regions may serve as regulatory sequences in the pre-mRNA
as exonic splicing enhancer
s or exonic splicing silencer
Untranslated regions (UTRs) are sections of the mRNA before the start codon and after the stop codon that are not translated, termed the five prime untranslated region
(5' UTR) and three prime untranslated region
(3' UTR), respectively. These regions are transcribed with the coding region and thus are exon
ic as they are present in the mature mRNA. Several roles in gene expression have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and translational efficiency
. The ability of a UTR to perform these functions depends on the sequence of the UTR and can differ between mRNAs. Genetic variants in 3' UTR have also been implicated in disease susceptibility because of the change in RNA structure and protein translation.
The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity for RNA degrading enzymes called ribonuclease
s and for ancillary proteins that can promote or inhibit RNA degradation. (See also, C-rich stability element
Translational efficiency, including sometimes the complete inhibition of translation, can be controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by influencing the ribosome's ability to bind to the mRNA. MicroRNA
s bound to the 3' UTR
also may affect translational efficiency or mRNA stability.
Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are needed in a particular region of the cell can also be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation.
Some of the elements contained in untranslated regions form a characteristic secondary structure
when transcribed into RNA. These structural mRNA elements are involved in regulating the mRNA. Some, such as the SECIS element
, are targets for proteins to bind. One class of mRNA element, the riboswitch
es, directly bind small molecules, changing their fold to modify levels of transcription or translation. In these cases, the mRNA regulates itself.
The 3' poly(A) tail is a long sequence of adenine
nucleotides (often several hundred) added to the 3' end
of the pre-mRNA. This tail promotes export from the nucleus and translation, and protects the mRNA from degradation.
Monocistronic versus polycistronic mRNA
An mRNA molecule is said to be monocistronic when it contains the genetic information to translate
only a single protein
chain (polypeptide). This is the case for most of the eukaryotic
On the other hand, polycistronic mRNA carries several open reading frame
s (ORFs), each of which is translated into a polypeptide. These polypeptides usually have a related function (they often are the subunits composing a final complex protein) and their coding sequence is grouped and regulated together in a regulatory region, containing a promoter
and an operator
. Most of the mRNA found in bacteria
is polycistronic, as is the human mitochondrial genome
. Dicistronic or bicistronic mRNA encodes only two protein
In eukaryotes mRNA molecules form circular structures due to an interaction between the eIF4E
and poly(A)-binding protein
, which both bind to eIF4G
, forming an mRNA-protein-mRNA bridge. Circularization is thought to promote cycling of ribosomes on the mRNA leading to time-efficient translation, and may also function to ensure only intact mRNA are translated (partially degraded mRNA characteristically have no m7G cap, or no poly-A tail).
Other mechanisms for circularization exist, particularly in virus mRNA. Poliovirus
mRNA uses a cloverleaf section towards its 5' end to bind PCBP2, which binds poly(A)-binding protein
, forming the familiar mRNA-protein-mRNA circle. Barley yellow dwarf virus
has binding between mRNA segments on its 5' end and 3' end (called kissing stem loops), circularizing the mRNA without any proteins involved.
RNA virus genomes (the + strands of which are translated as mRNA) are also commonly circularized. During genome replication the circularization acts to enhance genome replication speeds, cycling viral RNA-dependent RNA polymerase much the same as the ribosome is hypothesized to cycle.
Different mRNAs within the same cell have distinct lifetimes (stabilities). In bacterial cells, individual mRNAs can survive from seconds to more than an hour; in mammalian cells, mRNA lifetimes range from several minutes to days. The greater the stability of an mRNA the more protein may be produced from that mRNA. The limited lifetime of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs. There are many mechanisms that lead to the destruction of an mRNA, some of which are described below.
Prokaryotic mRNA degradation
In general, in prokaryotes the lifetime of mRNA is much shorter than in eukaryotes. Prokaryotes degrade messages by using a combination of ribonucleases, including endonucleases, 3' exonucleases, and 5' exonucleases. In some instances, small RNA molecules
(sRNA) tens to hundreds of nucleotides long can stimulate the degradation of specific mRNAs by base-pairing with complementary sequences and facilitating ribonuclease cleavage by RNase III
. It was recently shown that bacteria also have a sort of 5' cap
consisting of a triphosphate on the 5' end
. Removal of two of the phosphates leaves a 5' monophosphate, causing the message to be destroyed by the exonuclease RNase J
, which degrades 5' to 3'.
Eukaryotic mRNA turnover
Inside eukaryotic cells, there is a balance between the processes of translation
and mRNA decay. Messages that are being actively translated are bound by ribosome
s, the eukaryotic initiation factor
, and poly(A)-binding protein
. eIF-4E and eIF-4G block the decapping enzyme ( DCP2
), and poly(A)-binding protein blocks the exosome complex
, protecting the ends of the message. The balance between translation and decay is reflected in the size and abundance of cytoplasmic structures known as P-bodies
The poly(A) tail
of the mRNA is shortened by specialized exonucleases that are targeted to specific messenger RNAs by a combination of cis-regulatory sequences on the RNA and trans-acting RNA-binding proteins. Poly(A) tail removal is thought to disrupt the circular structure of the message and destabilize the cap binding complex
. The message is then subject to degradation by either the exosome complex
or the decapping complex
. In this way, translationally inactive messages can be destroyed quickly, while active messages remain intact. The mechanism by which translation stops and the message is handed-off to decay complexes is not understood in detail.
AU-rich element decay
The presence of AU-rich element
s in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these sequences and stimulate poly(A)
tail removal. Loss of the poly(A) tail is thought to promote mRNA degradation by facilitating attack by both the exosome complex
and the decapping complex
. Rapid mRNA degradation via AU-rich element
s is a critical mechanism for preventing the overproduction of potent cytokines such as tumor necrosis factor (TNF) and granulocyte-macrophage colony stimulating factor (GM-CSF). AU-rich elements also regulate the biosynthesis of proto-oncogenic transcription factors like c-Jun
Nonsense mediated decay
Eukaryotic messages are subject to surveillance by nonsense mediated decay
(NMD), which checks for the presence of premature stop codons (nonsense codons) in the message. These can arise via incomplete splicing, V(D)J recombination
in the adaptive immune system
, mutations in DNA, transcription errors, leaky scanning
by the ribosome causing a frame shift
, and other causes. Detection of a premature stop codon triggers mRNA degradation by 5' decapping, 3' poly(A)
tail removal, or endonucleolytic cleavage
Small interfering RNA (siRNA)
s, small interfering RNA
s (siRNAs) processed by Dicer
are incorporated into a complex known as the RNA-induced silencing complex
or RISC. This complex contains an endonuclease
that cleaves perfectly complementary messages to which the siRNA binds. The resulting mRNA fragments are then destroyed by exonuclease
s. siRNA is commonly used in laboratories to block the function of genes in cell culture. It is thought to be part of the innate immune system as a defense against double-stranded RNA viruses.
MicroRNAs (miRNAs) are small RNAs that typically are partially complementary to sequences in metazoan messenger RNAs. Binding of a miRNA to a message can repress translation of that message and accelerate poly(A) tail removal, thereby hastening mRNA degradation. The mechanism of action of miRNAs is the subject of active research.
Other decay mechanisms
There are other ways by which messages can be degraded, including non-stop decay
and silencing by Piwi-interacting RNA
(piRNA), among others.
Full length mRNA molecules have been proposed as therapeutics since the beginning of the biotech era but there was little traction until the 2010s, when Moderna Therapeutics
was founded and managed to raise almost a billion dollars in venture funding in its first three years.
Theoretically, the administered mRNA sequence can cause a cell to make a protein, which in turn could directly treat a disease or could function as a vaccine
; more indirectly the protein could drive an endogenous stem cell
to differentiate in a desired way.
Ribonucleic acid (RNA) is produced from deoxyribonucleic acid (DNA) in a process called transcription. RNA is primarily used as the code to make proteins and amino acids. Errors in protein or amino acid production can result in diseases such as cystic fibrosis or phenylketonuria. Therapies are now being developed that directly target RNA.https://www.khanacademy.org/science/biology/gene-expression-central-dogma/central-dogma-transcription/v/rna-transcription-and-translation
The primary challenges of RNA therapy center on delivering the RNA to directed cells, more even than determining what sequence to deliver. Naked RNA sequences will naturally degrade after preparation; they may trigger the body's immune system
to attack them as an invader; and they are impermeable
to the cell membrane
. Once within the cell, they must then leave the cell's transport mechanism to take action within the cytoplasm
, which houses the ribosomes
that direct manufacture of proteins
As opposed to targeting proteins with small chemicals or large compounds, drugs that target RNA hold the potential to treat numerous diseases resistant to conventional medicines. https://www.cbinsights.com