Translation2

Question 1

Why is translation important?

Answer 1

Because proteins are necessary for life as we know it, translation is essential. It must be rapid and precise. In some ways, one might think of translation as being “harder” than transcription because the nucleotide code must be read and then expressed as an amino acid sequence. Cells devote a lot of energy to it. Four high-energy bonds are used for each peptide bond that is made, and the translation machinery can make up a substantial percentage of the material in the cell. Also, many pathways exist to regulate translation which again speaks to how important it is for translation to occur correctly and efficiently. The basic processes of translation differ only a little across life, they are fundamental and conserved.

Question 2

What makes up the machinery of protein synthesis?

Answer 2

The key players are: messengerRNA(mRNA), Transfer RNAs (tRNA), Aminoacyl tRNA synthetases, ribosome, Initiation factors, Elongation factors and their partners, and Termination/recycling factors

Question 3

messengerRNA(mRNA)

Answer 3

contains the nucleotide sequence that encodes the protein. The protein that is encoded in each mRNA is written in three-nucleotide “codons.” The codons do not overlap. There are 43=64 possible codons, all are used. AUG is used as the “start” codon, and there are several used as “stop” codons. Several different codons can encode the same amino acid, but the frequency by which codons are used is not random and can vary between organisms.

Question 4

Transfer RNAs (tRNA)

Answer 4

adapters that “read” the message and deliver the right amino acid. tRNAs base-pair directly to the codons in the mRNA though the the tRNA’s anticodon loop. Thus, each codon is recognized by a specific type of tRNA. At the other end of the tRNA in the acceptor stem that has the amino acid attached that matches the anticodon. Some tRNAs can recognize more than one codon, due to wobble-pairing at the third location in the codon. The amount of each type of tRNA in the cell varies, usually matching codon frequency. The function of tRNA is dictated by its three-dimensional folded structure.

Question 5

Aminoacyl tRNA synthetases

Answer 5

protein enzymes that put the right amino acid on the right tRNA. These enzymes are very important in that each identifies the right tRNA and puts on the correct amino acid. Each amino acid/tRNA has its own synthetase associated with it (e.g. valyl-tRNA synthetase puts a valine on a val-tRNA).

Question 6

ribosome

Answer 6

the platform that brings it all together and contains the catalytic center. The ribosome is a massive machine containing both RNA and many proteins. The bulk of it is RNA. In all of life, it contains two subunits: in bacteria these are the 30S and 50S, and in eukaryotic these are the 40S and 60S. In a fully assembled ribosome, the mRNA and tRNA pass between the two subunits; there are three tRNA binding sites: the A, P, and E sites. The small subunit has the decoding groove through which the mRNA passes and the tRNAs read the message. The large subunit contains the catalytic center (the peptidyl transferase center, PTC), which appears to be made entirely of RNA and thus the ribosome is a ribozyme (uses RNA to perform catalysis). Many antibiotics target the ribosome.

Question 7

Initiation factors

Answer 7

proteins that bring the ribosome to the message RNA and assist in getting the machinery assembled. There are three of these in bacteria, over a dozen in eukaryotes.

Question 8

Elongation factors and their partners

Answer 8

proteins that deliver tRNAs and move the ribosome down the message.

Question 9

Termination/recycling factors

Answer 9

proteins that deliver tRNAs and move the ribosome down the message.

Question 10

Initiation

Answer 10

getting the machinery assembled in the right place and thus setting the reading frame. Initiation is the step that differs the most between bacteria and eukaryotes. However, in both the goal is the same: assemble a ribosome with the start codon (AUG) and initiator methionine tRNA in the P-site, ready to receive the next aa-tRNA in the A-site.

Question 11

initiator methionine tRNA

Answer 11

initiates translation in eukaryotes, homologous to N-Formylmethionine (fMet)

Question 12

N-Formylmethionine (fMet)

Answer 12

a derivative of the amino acid methionine in which a formyl group has been added to the amino group. It is specifically used for initiation of protein synthesis for bacterial and organellar genes, and may be removed post-translationally. It is located at the N-terminus of the growing polypeptide. fMet is delivered to the ribosome (30S) - mRNA complex by a specialized tRNA (tRNAfMet) which has a 3’-UAC-5’ anticodon that is capable of binding with the 5’-AUG-3’ start codon located on the mRNA.

Question 13

Shine- Dalgarno sequence

Answer 13

a ribosomal binding site in prokaryotic mRNA, generally located around 8 bases upstream of the start codon AUG.[1] The RNA sequence helps recruit the ribosome to the mRNA to initiate protein synthesis by aligning the ribosome with the start codon. The ribosome recognises a purine-rich sequence (AGGAGGU) in the region upstream of the correct initiator AUG found in the ribosome binding sites

Question 14

untranslated region

Answer 14

refers to either of two sections, one on each side of a coding sequence on a strand of mRNA. If it is found on the 5’ side, it is called the 5’ UTR (or leader sequence), or if it is found on the 3’ side, it is called the 3’ UTR (or trailer sequence).

Question 15

5’ untranslated region (5′ UTR)

Answer 15

The region of an mRNA that is directly upstream from the initiation codon. The elements of a eukaryotic and prokaryotic 5′ UTR differ greatly. The prokaryotic 5′ UTR contains a ribosome binding site (RBS), also known as the Shine Dalgarno sequence (AGGAGGU) which is usually 3-10 base pairs upstream from the initiation codon. Meanwhile the eukaryotic 5′ UTR contains the Kozak consensus sequence (ACCAUGG), which contains the initiation codon. The eukaryotic 5′ UTR also contains cis-acting regulatory elements called upstream open reading frames (uORFs) and upstream AUGs and termination codons (uAUGs), which have a great impact on the regulation of translation.

Question 16

three prime untranslated region (3’-UTR)

Answer 16

Regulatory regions within the 3’-untranslated region can influence polyadenylation, translation efficiency, localization, and stability of the mRNA. The 3’-UTR contains both binding sites for regulatory proteins as well as microRNAs (miRNAs). The 3’-UTR also has silencer regions which bind to repressor proteins and will inhibit the expression of the mRNA. Furthermore, the 3’-UTR contains the sequence AAUAAA that directs addition of several hundred adenine residues called the poly(A) tail to the end of the mRNA transcript.

Question 17

Translation in bacteria

Answer 17

in bacteria, there is no cap, no poly-A tail, and generally no UTRs. Several proteins can be encoded on a single mRNA (polycistronic). 1) Initiation factor proteins IF1 and IF3 bind the 30S subunit. The mRNA binds the 30S subunit using the Shine-Dalgaro sequence. This places the start AUG codon in the subunit’s P-site. 2) IF2 delivers a special “initiator” formylmethionine tRNA to the P-site to pair with the AUG codon. 3) GTP hydrolysis on IF2 leads to release of all the initiation factors and binding of the 50S subunit. This results in a 70S ribosome with the next codon to be read placed in the A-site.

Question 18

Implication of the mechanism of initiation in bacteria

Answer 18

Initiation can occur at internal AUG codons in prokaryotic mRNA, so messages can be polycistronic (many messages strung together on a single RNA). Bacterial genes are often expressed in groups as an operon. Example: The lac operon in E. coli contains many AUGs, but only those with an associated SD sequence are sites of translation initiation.

Question 19

translation in eukaryotes

Answer 19

initiation factor (eIF) 4E is required to bind to the 7-methyl guanosine cap on the 5’ end of the mRNA. This leads to binding of many other eIFs (4G, 4A, 4B, etc.) and eventually to binding of the small ribosomal subunit, which itself is bound by several factors (eIF3, eIF1A, eIF1, eIF2, etc.). The ribosome then scans down the message to find the AUG start codon. At that point, the large subunit can join the small, the factors are released, and the goal of initiation has been achieved. In addition to the canonical cap-dependent process, there is a cap-independent process in eukaryotes that is driven by specific RNA sequences and structures called internal ribosome entry sites. The majority of eukaryotic mRNAs are translated by a cap-dependent, scanning mechanism. The greater complexity allows for regulation of the process in many ways.

Question 20

initiation factor (eIF) 4E

Answer 20

EIF4E is a eukaryotic translation initiation factor involved in directing ribosomes to the cap structure of mRNAs. The EIF4E polypeptide is the rate-limiting component of the eukaryotic translation apparatus and is involved in the mRNA-ribosome binding step of eukaryotic protein synthesis. eIF4E binds the first nucleotide on the 5’ end of an mRNA molecule (known as the cap): a 7 methyl guanosine (m7G),

Question 21

7-methyl guanosine cap

Answer 21

The 5′ cap is found on the 5′ end of an mRNA molecule and consists of a guanine nucleotide connected to the mRNA via an unusual 5′ to 5′ triphosphate linkage. This guanosine is methylated on the 7 position directly after capping in vivo by a methyl transferase. This structure is involved in several cellular processes including enhanced translational efficiency, splicing, mRNA stability, and RNA nuclear export.

Question 22

eIF4G

Answer 22

a protein involved in bringing mRNA to the ribosome for translation, eIF4G strongly associates with the protein that directly binds the mRNA cap: eIF4E. It is a scaffolding protein that also directly associates with eIF3

Question 23

eIF4A

Answer 23

The mRNA cap is bound by eIF4E, eIF4G acts as a scaffold for the complex whilst the ATP-dependent RNA helicase eIF4A processes the secondary structure of the mRNA 5’ UTR to render it more conducive to ribosomal binding and subsequent translation. Together these three proteins are referred to as eIF4F.

Question 24

eIF4B

Answer 24

Required for the binding of mRNA to ribosomes. Functions in close association with EIF4-F and EIF4-A. Binds near the 5’-terminal cap of mRNA in presence of EIF-4F and ATP. Promotes the ATPase activity and the ATP-dependent RNA unwinding activity of both EIF4-A and EIF4-F

Question 25

eIF3

Answer 25

is associated with the small ribosomal subunit, and plays a role in keeping the large ribosomal subunit from prematurely binding. eIF3 also interacts with the eIF4F complex

Question 26

eIF1A

Answer 26

Binds to the small ribosomal subunit, is essential for transfer of the initiator Met-tRNAf to 40 S ribosomal subunits in the absence of mRNA to form the 40 S preinitiation complex

Question 27

eIF1

Answer 27

Binds to the small ribosomal subunit

Question 28

eIF2

Answer 28

eIF2 is a GTP-binding protein responsible for bringing the initiator tRNA to the P-site of the pre-initiation complex. It has specificity for the methionine-charged initiator tRNA, which is distinct from other methionine-charged tRNAs specific for elongation of the polypeptide chain. Once it has placed the initiator tRNA on the AUG start codon in the P-site, it hydrolyzes GTP into GDP, and dissociates. This hydrolysis, also signals for the dissociation of eIF3, eIF1, and eIF1A, and allows the large subunit to bind. This signals the beginning of elongation. When large numbers of eIF2 are phosphorylated, protein synthesis is inhibited. This would occur if there is amino acid starvation or there has been a virus infection.

Question 29

Poly(A)-binding protein (PAB or PABP)

Answer 29

a RNA-binding protein which binds to the poly(A) tail of mRNA. binds to the initiation factor eIF-4G via its C-terminal domain. EIF-4G is bound to eIF-4E, another initiation factor bound to the 5’ cap on the 5’ end of mRNA. This binding forms the characteristic loop structure of eukaryotic protein synthesis.

Question 30

cap-dependent process

Answer 30

Initiation of translation usually involves the interaction of certain key proteins with a special tag bound to the 5’-end of an mRNA molecule, the 5’ cap, as well as with the 5’ UTR. The protein factors bind the small ribosomal subunit (also referred to as the 40S subunit), and these initiation factors hold the mRNA in place.

Question 31

internal ribosome entry sites

Answer 31

Many viruses use IRESs to initiation translation after they shut down host cell cap-dependent synthesis. In addition, there is strong evidence that some eukaryotic mRNAs use IRESs.

Question 32

aminoacyl tRNA synthetases

Answer 32

an enzyme that catalyzes the esterification of a specific cognate amino acid or its precursor to one of all its compatible cognate tRNAs to form an aminoacyl-tRNA. The synthetase first binds ATP and the corresponding amino acid (or its precursor) to form an aminoacyl-adenylate, releasing inorganic pyrophosphate (PPi). The adenylate-aaRS complex then binds the appropriate tRNA molecule, and the amino acid is transferred from the aa-AMP to the 3’-end.

Question 33

IF2

Answer 33

is a prokaryotic initiation factor.binds to an initiator tRNA and controls the entry of that tRNA into the ribosome. IF2, bound to GTP, binds to the 30S P site. After associating with the 30S subunit, fMet-tRNAf binds to the IF2 then IF2 transfers the tRNA into the partial P site. When the 50S subunit joins, it hydrolyzes GTP to GDP and Pi, causing a conformational change in the IF2 that causes IF2 to release and allow the 70S subunit to form.

Question 34

aminoacyl site

Answer 34

a binding site in a ribosome for charged t-RNA molecules during protein synthesis. One of three such binding sites, the A-site is the first location the t-RNA binds during the protein synthesis process

Question 35

peptidyl site

Answer 35

the ribosomal site most frequently occupied by peptidyl-tRNA, i.e. the tRNA carrying the growing peptide chain.

Question 36

exit site

Answer 36

the ribosomal site harbouring decylated tRNA on transit out from the ribosome.

Question 37

peptidyl transferase center

Answer 37

an aminoacyltransferase as well as the primary enzymatic function of the ribosome, which forms peptide bonds between adjacent amino acids using tRNAs during the translation process of protein biosynthesis. is contained the 23S/28S rRNA on the large ribosomal subunit. Peptidyl transferase activity is carried out by the ribosome. Peptidyl transferase activity is not mediated by any ribosomal proteins but by ribosomal RNA (rRNA), a ribozyme. This RNA relic is the most significant piece of evidence supporting the RNA World hypothesis.

Question 38

Small ribosome subunit

Answer 38

The message is decoded on the small subunit

Question 39

large ribosome subunit

Answer 39

The amino acids join to form the peptide chain in the peptidyl transferase center of the large subunit

Question 40

Elongation in eukaryotes

Answer 40

1) Peptide bond formation is catalyzed by in the peptidyl transferase center (PTC). The energy for peptide bond formation comes from the ATP used in tRNA charging. Peptide bond formation results in a transfer of the nascent peptide from the P-site tRNA to the A-site tRNA. 2) After the peptide bond is made, EF2 and GTP hydrolysis triggers movement of the mRNA and tRNAs exactly one codon in the 3’ direction. 3) The result is the deacylated tRNA now in the E site, and the tRNA with the peptide chain attached in the P site. 4) Now, the cycle can continue when a new aminoacylated tRNA, bound to EF1a, moves into the vacant A site.

Question 41

EF-Tu

Answer 41

(elongation factor thermo unstable) is one of the prokaryotic elongation factors. helps the aminoacyl-tRNA move onto a free site on the ribosome. In the cytoplasm, EF-Tu binds an aminoacylated, or charged, tRNA molecule. This complex enters the ribosome. Homologous to EF1A in eukaryotes

Question 42

Energy used in elongation

Answer 42

Each peptide bond requires spending 4 high- energy bonds: To charge tRNA: 2 (ATP->AMP), To deliver aa-tRNA to A site: 1 (GTP->GDP), Translocation: 1 (GTP->GDP)

Question 43

Termination

Answer 43

Reading a stop codon at the end of the sequence, ending elongation, and dissociating the subunits. This is accomplished when a stop codon is detected by a recycling factor in the A-site. Recycling factors are proteins that fit into the same space as a tRNA, but when they do, they trigger the termination of the peptide chain, and a series of events that lead to release of the peptide and dissociation of the subunits. The protein then goes off to fold, receive any modifications, etc.

Question 44

Recycling

Answer 44

getting the ribosomal subunits ready to be used again in initiation.

Question 45

missense mutations

Answer 45

Mutation resulting in an amino acid change. The codon is changed so now is encodes a different amino acid.

Question 46

Silent mutation

Answer 46

The codon is changed, but the same amino acid is encoded (mutation in a wobble position).

Question 47

Frame-shift mutation

Answer 47

An addition or deletion of a nucleotide shifts the reading frame.

Question 48

nonsense mutation

Answer 48

Mutation resulting in a premature stop codon. The mutation changes the codon from an amino acid encoding one to a stop codon, resulting in a truncated protein.

Question 49

sense mutation

Answer 49

Mutation resulting in a removed stop codon. Opposite of a nonsense mutation – now the stop codon encodes an amino acid and the ribosome keeps going.

Question 50

Hemoglobin Wayne and Hemoglobin Constant Spring

Answer 50

result from a frame- shift mutation and a sense mutation, respectively. Both lead to proteins with longer C- terminal “tails.” They are associated with chronic anemia.

Question 51

How can translation be regulated or altered?

Answer 51

1. Regulation by varying the mRNA structure and sequence. 2. Regulation by altering the function of initiation factors. 3. Regulation by protein binding to the mRNA.

Question 52

Regulation by varying the mRNA structure and sequence.

Answer 52

some codons appear with different frequency, and tRNA levels vary. Hence, it is possible that a change in the sequence of an mRNA might give rise to a codon that still encodes the same protein, but for which the tRNA is “rare.” This could slow the production of the protein. Also, the presence of structures upstream of the start codon that inhibit scanning of the ribosome (in eukaryotes) or occlude the Shine Dalgano sequence (in bacteria). These would decrease the rate of initiation on that specific message.

Question 53

IRES RNAs

Answer 53

can drive a cap-independent pathway o fribosome recruitmentand initiation in eukaryotes. IRESs are used by viruses, but it also appears their functional can be modulated based on the cellular conditions and thus they are probably very important for regulating gene expression.

Question 54

Frameshifting

Answer 54

occurs when a specific RNA sequence and structure in the middle of the coding part of the RNA results in the ribosome slipping into a different reading frame a certain percentage of the time. This is an example of “recoding.”

Question 55

RNA editing

Answer 55

This occurs when the transcribed mRNA is modified specifically in such a way that the coding is affected. This can be tissue specific, so the same gene encoded in DNA can be use to produce two different proteins. apoB is a good examples.

Question 56

Kozak consensus sequence

Answer 56

This sequence on an eukaryotic mRNA molecule is recognized by the ribosome as the translational start site, from which a protein is coded by that mRNA molecule. The ribosome requires this sequence, or a possible variation to initiate translation. During scanning, different AUGs have different efficiencies of recognition– “Kozak sequence”. Different AUG start codons of different “strengths” on a single RNA can give rise to different patterns of protein expression.

Question 57

Regulation by altering the function of initiation factors

Answer 57

In eukaryotes, the cap binding protein (eIF4E) can be bound by 4E-binding proteins (4E-BPs) that sequester it and blocks its function. When these proteins are phosphorylated, they do not bind to 4E and this allows cap-dependent translation initiation. However, under some conditions (for example, stress), the 4E-BPs are dephosphorylated, they bind to 4E, and they block its function. This can also be induced by the drug rapamycin. This results in a shut-down of much of translation. However, note that some messages are less dependent on 4E (e.g. IRES-containing messages), and these could still be translated. The role of eIF4E levels and activity in cancer is a hot topic.

Question 58

eIF2-alpha

Answer 58

is critical for the steps that lead to binding of the initiator tRNA to the ribosome. When eIF2-alpha is phosphorylated, its activity is inhibited and this blocks initiation. eIF2-alpha can be phosphorylated by several pathways. One is induced by interferon, which is produced when a cell is infected by a virus. Hence, shutting down translation is a response to viral infection. Also, many other cellular stresses lead to phosphorylation of eIF2-alpha and so this is an important way for the cell to regulate protein synthesis during certain conditions.

Question 59

Regulation by protein binding to the mRNA.

Answer 59

The 3’ and 5’ UTRs are eukaryotic mRNAs are full of sites that can be bound by specific proteins. These binding interactions can affect everything from mRNA stability, the localization of the mRNA, the degree to which ribosomes bind, etc. A good example of protein binding to specific structures in an mRNA is that of iron metabolism.

Question 60

poly-A tail

Answer 60

the presence, absence, and length of the poly-A tail on the extreme 3’ end of the RNA can affect the degree to which it is translated, presumably because of protein binding. During oocyte maturation, mRNAs with short poly-A tiles are inherited from the mother but are quiescent. At the right time in development, their poly-A tails grow and they become translationally active.

Question 61

Iron metabolism

Answer 61

This mechanism involves protein binding to specific mRNAs, and changes in the binding pattern that occur when iron levels change. Iron (Fe) is required for the activity of many biological molecules (e.g. hemoglobin, reductases). High levels of iron are very toxic. Cells must establish and maintain a balance, keeping enough
iron to do its many jobs, but not enough to poison the cell. Under high Fe conditions, IRE-BPs are bound to Fe and CAN’T bind the IRE RNA. Under conditions of low Fe, IRE-BPs are not bound to Fe and CAN bind the IRE RNA.

Question 62

Transferrin

Answer 62

iron-binding blood plasma glycoproteins that control the level of free iron in biological fluids.

Question 63

Transferrin receptor (TfR)

Answer 63

a carrier protein for transferrin. It is needed for the import of iron into the cell and is regulated in response to intracellular iron concentration. It imports iron by internalizing the transferrin-iron complex through receptor-mediated endocytosis. Protein level goes up with low iron. Protein level goes down with high iron.

Question 64

Ferritin

Answer 64

serves to store iron in a non-toxic form, to deposit it in a safe form, and to transport it to areas where it is required. The function and structure of the expressed ferritin protein varies in different cell types. This is controlled primarily by the amount and stability of mRNA. mRNA concentration is further tweaked by changes to how it is stored and how efficiently it is transcribed. The presence of iron itself is a major trigger for the production of ferritin.

Question 65

Iron-responsive element (IRE)

Answer 65

a short conserved stem-loop which is bound by iron response proteins. The IRE is found in UTRs (untranslated regions) of various mRNAs whose products are involved in iron metabolism. For example, the mRNA of ferritin (an iron storage protein) contains one IRE in its 5’ UTR. When iron concentration is low, IRPs bind the IRE in the ferritin mRNA and cause reduced translation rates. In contrast, binding to multiple IREs in the 3’ UTR of the transferrin receptor (involved in iron acquisition) leads to increased mRNA stability.

Question 66

Iron Response Binding Proteins (IRE-BPs) 1 & 2

Answer 66

the IRE-BP binds to the IREs of ferritin and transferrin receptor mRNA. But, when iron binds to the IRE-BP, the IRE-BP changes shape with the result that the IRE-BPs can no longer bind the ferritin mRNA. This liberates the mRNA to direct the cell to make more ferritin. In other words, when there is high iron in the cell, the iron itself causes the cell to produce more iron storage molecules.

Question 67

The cap-independent initiation (IRES driven)

Answer 67

What differentiates cap-independent translation from cap-dependent translation is that cap-independent translation does not require the ribosome to start scanning from the 5’ end of the mRNA cap until the start codon. The ribosome can be trafficked to the start site by ITAFs (IRES trans-acting factors) bypassing the need to scan from the 5’ UTR. This method of translation has been recently discovered, and has found important in conditions that require the translation of specific mRNAs, despite cellular stress or the inability to translate most mRNAs. Examples include factors responding to apoptosis, stress-induced responses. In some cases, the virus produces a protease that cleaves eIF4G, shutting down cap-dependent translation. The virus can continue using an IRES. It is also utilized by a subset of eukaryotic mRNAs

Question 68

mechanistic target of rapamycin (mTOR)

Answer 68

is a serine/threonine protein kinase that regulates cell growth, cell proliferation, cell motility, cell survival, protein synthesis, and transcription. MTOR integrates the input from upstream pathways, including insulin, growth factors, and amino acids. mTOR also senses cellular nutrient, oxygen, and energy levels

Question 69

EIF4EBP1 (Eukaryotic translation initiation factor 4E binding protein 1)

Answer 69

It act by binding to the mRNA cap-binding protein eukaryotic initiation factor 4E (eIF4E), in competition with another initiation factor, eIF4G, that is essential for polypeptide chain initiation. Thus the availability of eIF4E for translation of cap-dependent mRNAs is limited by the extent to which this factor is sequestered by the 4E-BPs. reversibly phosphorylated at multiple sites, in response to several physiological signals that promote translation. Such phosphorylations lower the affinity of 4E-BP1 for eIF4E and result in the dissociation of the two proteins, thereby enhancing the level of active eIF4E and promoting the translation of capped mRNAs. Conversely, physiological stresses and other conditions that inhibit translation.

Question 70

Interferons (IFNs)

Answer 70

they are antiviral agents and they modulate functions of the immune system. A virus-infected cell releases viral particles that can infect nearby cells. However, the infected cell can prime neighboring cells for a potential infection by the virus by releasing interferon. In response to interferon, cells produce large amounts of an enzyme known as protein kinase R (PKR). This enzyme phosphorylates a protein known as eIF-2 in response to new viral infections; the phosphorylated eIF-2 forms an inactive complex with another protein, called eIF2B, to reduce protein synthesis within the cell. It also induces the activity of 2-5A synthase

Question 71

2-5A synthase

Answer 71

t is an antiviral enzyme that counteracts viral attack by degrading viral RNA. The enzyme uses ATP in 2’-specific nucleotidyl transfer reactions to synthesize 2’-5’-oligoadenylates, which activate latent ribonuclease, resulting in degradation of viral RNA and inhibition of virus replication.

Question 72

eIF-2a

Answer 72

The protein encoded by this gene is the alpha subunit of the translation initiation factor eIF2 complex which catalyzes the first regulated step of protein synthesis initiation, promoting the binding of the initiator tRNA to 40S ribosomal subunits. eIF-2a phosphorylation inhibits ternary complex formation (no initiator tRNAmet delivered to ribosome) in response to cellular stress