Translation (biology)

As translation, the synthesis of proteins of living organisms in the cells (see protein ) identified by means of the copied to mRNA molecules genetic information. Translation as an integral part of the process of gene expression, the transcription downstream ( complementary copy of the DNA - information to individual mRNA strands ), and is carried out in living cells of specific structures, the ribosomes.

• 4.1 posttranslational protein transport
• 4.2 cotranslational protein transport

• 5.1 Example of a regulation of translation of ribosomal proteins

General sequence

The according to the genetic code of a section of DNA formed and optionally processed mRNA ( "m" for engl. " Messenger", messenger ) contains information on the structure of a protein. This information will be used during the translation, to synthesize the corresponding protein. It encode three consecutive nucleotides of the mRNA which are also called base triplets or codons a particular amino acid (see: genetic code). The protein is built up sequentially from the series- encoded amino acids. In this case processes a ribosome mRNA in the 5 ' to 3' direction, the direction in which the RNA was also synthesized.

For this process as amino acid "Transporter" the tRNA (, t ' for engl. Transfer, transmission ) are necessary. This can be loaded at one end, the anticodon, dock at the appropriate codon on the mRNA and at its other end by the aminoacyl- tRNA synthetases with the matching exactly for this amino acid codon.

The ribosome brings the mRNA and tRNA, which has included an amino acid together so that on the mRNA at a particular codon - as a matching counterpart - attaches a complementary anticodon of tRNA. The process begins at the location of the mRNA, which carries the start codon with the sequence August (adenine, uracil, guanine). A second tRNA, which also carries an amino acid that sits next to the first tRNA to the mRNA. The two hanging on the tRNAs amino acids are linked to a peptide bond, and the first amino acid tRNA without leaving the ribosome. The right to the next codon tRNA now attaches to the mRNA. Their amino acid is linked to the already existing amino acid chain and extended this by another member. This process continues so that an increasingly long chain of amino acids forms. The ribosome, which catalyzes the process, always moves to a codon on the mRNA on, namely, until the information of the mRNA has been completely processed. A stop codon ( zBUGA ) is located at this site in the mRNA. At this none of the existing tRNA molecule types can dock.

This protein newly formed by the concatenation of the amino acid detaches from the ribosome and then folds mostly so that a complex spatial structure is formed ( secondary and tertiary structure ). It may Connects Even with other proteins to the parent quaternary structures.

MRNA is repeatedly read out in the control, until it is decomposed by nuclease into their building blocks which ribonucleotides. In eukaryotes, the durability is increased by post-transcriptional modifications in the core.

Biochemical sequence

There are ( in humans) only 41 different tRNA species, which is not equivalent to the 64 triplets. The third position of the codon is variable with many amino acids. For this purpose, the term wobble hypothesis will be used. The tRNA has the familiar trefoil, due to the intramolecular base pairing of complementary nucleotides in the secondary structure. However, this does not correspond to the three-dimensional tertiary structure, which is more like the letter L.

The acceptor arm 5'- and the 3'- end are combined. At the 3'- end, the post-transcriptional CCA binds the corresponding amino acid.

The anticodon loop is located opposite the secondary structure in the tertiary structure, this has the farthest distance to the acceptor arm. Three main bases - usually number 34, 35 and 36 - constitute the anticodon. Where position 34 constitutes the first base of the anticodon, which pairs with the third base of the codon.

The D - arm contains the unusual dihydrouridine.

The T- arm typically contains pseudouridine and cytosine.

The V- loop is variable, so differ among the tRNAs.

The loading of the tRNA with an amino acid is carried out by an aminoacyl -tRNA synthetase. For each amino acid there is a specific synthetase. The bases mentioned are usually valid, but are also variable.

Ribosomes and protein synthesis

The ribosome binding of aminoacyl-tRNA is made at the codons of mRNA and synthesis of proteins.

In the ribosomes, two subunits can ( each consisting of RNA and polypeptides ) are different, which exist separately at first. During translation they unite and form two functionally important regions, in which the tRNAs can bind: on the peptidyl site ( P site ) sits the tRNA with the growing protein chain to the aminoacyl site ( A-site ) binds the tRNA with the next amino acid to be joined. Another region is referred to as an exit site ( E site ) where they exit from the discharged tRNAs, the ribosome.

Initiation of translation in prokaryotes

For the start of the chain ( initiation), the cell requires a specific tRNA. This binds to the start codon AUG transfers in bacteria but formylmethionine ( fMet ), instead of the usual methionine. At the time of initiation ( in prokaryotes ) three initiation factors ( IF 1, IF 2, IF 3 ) play a role.

The small subunit ( 30S ) is at the beginning of a complex with the initiation factors 1 and 3 The purpose of the IF1 is the dissociation of the ( lying in a dynamic equilibrium ) Non initiator tRNA. The IF3 prevents together with the IF1 early binding of the two ribosomal subunits. IF2 of a G- protein binds GTP and undergoes a conformational change thereby obtains the possibility of the initiator tRNA bind fMet. This complex has the possibility of the mRNA and the small subunit binding. The bonding is effected by an interaction of the anti- Shine-Dalgarno sequence of 16S rRNA (ribosomal RNA and part of the 30S unit ) with the Shine- Dalgarno sequence on the mRNA. This is an upstream ( 5 'to the AUG codon located ), this non- coding sequence. This results in the detection of the start codon by the initiator tRNA. The completion of the initiation is initiated by GTP hydrolysis on IF2. It is for discharging the initiation factors and binding of the 50S subunit, whereby the 70S initiator complex is formed. The fMet -tRNA is located at the beginning of the translation already in the P site of the 50S subunit. The other two points, A and E are empty.

Elongation of the polypeptide chain

The elongation is the process of extension of the amino acid chain; they will be held on recognition and at the binding site of the ribosome. A single elongation step contains three steps: binding of charged tRNA, formation of the peptide bond and prepare for the next elongation step. This is repeated until a terminating codon is reached.

Termination

The end of the translation is obtained if the mRNA appears on one of the stop triplet UAG, UAA, or UGA. Since there is no appropriate codons for these tRNA in the cell that stops the translation.

Termination factors (release factors ) bind to the stop codon: RF1 at UAG and UAA or RF2 at UAA and UGA. The cleavage of the bond between the last amino acid and the tRNA last is the most interesting feature of the ribosome. The esters can not be broken by hydrolysis, since the region of the peptidyl transferase must be completely anhydrous in order to prevent spontaneous hydrolysis during elongation. The RF transported via a specific aa sequence (glycine -glycine- glutamine) exactly one molecule of water in the peptidyl transferase center. This can then with the aid of catalytic activity of the ribosome cleave the ester bond. This sequence is also found in eukaryotic RF. Dissociation of RF1/RF2 from ribosomes is catalyzed by the termination factor RF3.

Now the protein and the mRNA from the ribosome falls off, the decays back to its two subunits. The initiation factor IF3 maintains the dissociated state. Thus, the cycle anew begin.

Translation in eukaryotes

The translation in eukaryotes differs especially in the initiation of prokaryotic translation. Involved are therefore specific eukaryotic initiation factors. There is no fMet -tRNA, but an initiator Met -tRNA, which is not formylated in vivo. Also, there is no Shine- Dalgarno sequence on the mRNA. It is usually chosen first base triplet August (5'- seitigste ) of the mRNA codon. The binding of the 40S sub- unit is at the 5 ' cap structure of the mRNA. Then the ribosome goes in the 3 ' direction on the mRNA and searches along a August The success of the search for " reports" a successful pairs of the initiator Met -tRNA to the mRNA. Furthermore, the eukaryotic mRNA has to form complex secondary structures due to the processing and transport from the nucleus enough time. These must be broken up by helicases again.

Translation by membranes

Since most bacteria or prokaryotes are surrounded by several stable membranes have here developed some special mechanisms to synthesize proteins through membranes to the outside. Also in eukaryotes such mechanisms occur, since the organelles of a (double ) membrane are surrounded. There are two courses of action:

Posttranslational protein transport

In this method, the protein is completely assembled within the cell and prevent premature folding chaperones, wherein in the bacteria by a built-in "kink " in the threading of the protein is facilitated by the cell membrane. In eukaryotes, the posttranslational transport across the ER membrane has so far been shown only in yeast.

Cotranslational protein transport

In this way the ribosome is brought (mainly the endoplasmic reticulum ) by signaling proteins to the cell membrane. The resulting protein is then pushed through a special tunnel in the underlying field.

Regulation

Each protein required for survival of the cell is encoded in the genes. The amount required is, however, not directly encoded in the gene and also depending on environmental conditions, age and cell cycle.

Under the control of translation is understood, accordingly, various biochemical mechanisms that control the translation, that protein. This control is carried out similarly to the transcription: a repressor binds to the translation start point, thereby blocking the initiation of the process. In some cases, this involves the detection of specific structures in the mRNA. An important signal which controls by regulating the translation of cell growth and cell cycle, the mTOR signaling pathway.

In addition to the regulation of translation, the cell still has a few more options to control the amount of protein expressed by influencing previous links in the chain of information:

• Regulation of the transcription

Eukaryotic cells also:

• Regulation of the transport of the mRNA from the nucleus
• Alternative splicing

Example of regulation of translation of ribosomal proteins

The correct expression of ribosomal proteins is an interesting regulatory problem for the cell dar. Each ribosome contains about 50 specific proteins, all of which must be synthesized at the same rate. Furthermore, the rate of synthesis of proteins of the cell and the need for closely related ribosome with the cell growth. A change in the growth conditions quickly leads to an increase or decrease of the rate of synthesis of ribosomal components. For this, a regulation is needed.

The control of the genes for ribosomal proteins is facilitated by the organization in different operons, each containing up to 11 genes for ribosomal proteins.

The primary control is done at the level of translation. This can be detected for example, by the following experiment:

If by engineering additional copies of such operon introduced into the genome of a cell increases, accordingly, the amount of mRNA produced by transcription. Nevertheless, the rate of synthesis of the protein remains almost unchanged. Thus, the cell compensates for the increased amount of mRNA. Here ribosomal proteins act as repressors of their own translation.

For each operon may already synthesized a ribosomal protein binding to the mRNA of the operon. This binding site is located near one of the first genes of the operon. Characterized ribosomes will be prevented from binding to the mRNA, and to start the translation. Repression of translation of the first gene thus preventing expression of a portion or all of the remainder of the following genes.

This mechanism is very sensitive. A few do not, for example, prevent the formation of ribosomes consumed molecules of the protein L4 both the synthesis of this protein as well as the rest of the 10 ribosomal proteins in the same operon. This will therefore ensure that the proteins are not produced in large quantities which can not be completely consumed in the formation of ribosomes.

Such a protein can serve as both a ribosomal component and as a regulator of its own translation, could be explored with the binding sites with its own mRNA by comparison of the binding sites of the protein to the rRNA. Both binding sites are similar in their sequence and its secondary structure. Since the binding of the ribosomal proteins in the rRNA is stronger than that of the mRNA, translation is suppressed only when the demand for proteins for the production of ribosomes is covered.