Bacterial translation is the process by which messenger RNA is translated into proteins in bacteria.
Initiation of translation in bacteria involves the assembly of the components of the translation system, which are: the two ribosomal subunits (50S and 30S subunits); the mature mRNA to be translated; the tRNA charged with N-formylmethionine (the first amino acid in the nascent peptide); guanosine triphosphate (GTP) as a source of energy, and the three prokaryotic initiation factors IF1, IF2, and IF3, which help the assembly of the initiation complex. Variations in the mechanism can be anticipated. [1]
The ribosome has three active sites: the A site, the P site, and the E site. The A site is the point of entry for the aminoacyl tRNA (except for the first aminoacyl tRNA, which enters at the P site). The P site is where the peptidyl tRNA is formed in the ribosome. And the E site which is the exit site of the now uncharged tRNA after it gives its amino acid to the growing peptide chain. [1]
The majority of mRNAs in E. coli are prefaced with a Shine-Dalgarno (SD) sequence. The SD sequence is recognized by an complementary "anti-SD" region on the 16S rRNA component of the 30S subunit. In the canonical model, the 30S ribosome is first joined up with the three initiation factors, forming an unstable "pre-initiation complex". The mRNA then pairs up with this anti-SD region, causing it to form a double-stranded RNA structure, roughly positioning the start codon at the P site. An initiating tRNAfMet arrives and is positioned with the help of IF2, starting the translation. [1]
There are a lot of uncertainties even in the canonical model. The initiation site has been shown to be not strictly limited to AUG. Well-known coding regions that do not have AUG initiation codons are those of lacI (GUG) [2] and lacA (UUG) in the E. coli lac operon. [3] Two studies have independently shown that 17 or more non-AUG start codons may initiate translation in E. coli. [4] [5] Nevertheless, AUG seems to at least be the strongest initiation codon among all possibilities. [1]
The SD sequence also does not appear strictly necessary, as a wide range of mRNAs lack them and are still translated, with an entire phylum of bacteria (Bacteroidetes) using no such sequence. Simply SD followed by AUG is also not sufficient to initiate translation. It does, at least, function as a very important initiating signal in E. coli. [1]
When translating a polycistronic mRNA, a 70S ribosome ends translation at a stop codon. It is now shown that instead of immediately splitting into its two halves, the ribosome can "scan" forward until it hits another Shine–Dalgarno sequence and the downstream initiation codon, initiating another translation with the help of IF2 and IF3. [6] This mode is thought to be important for the translation of genes that are clustered in poly-cistronic operons, where the canonical binding mode can be disruptive due to small distances between neighboring genes on the same mRNA molecule. [7]
A number of bacterial mRNAs have no 5'UTR whatsoever, or a very short one. The complete 70S ribosome, with the help of IF2 (recruiting fMet-tRNA), [8] can simply start translating such a "leaderless" mRNA. [1]
A number of factors modify the efficiency of leaderless initiation. A 5' phosphate group attached to the start codon seems near-essential. [1] AUG is strongly preferred in E. coli, but not necessarily in other species. IF3 inhibits leaderless initiation. [1] A longer 5'UTR or one with significant secondary structure also inhibits leaderless initiation. [9]
Elongation of the polypeptide chain involves addition of amino acids to the carboxyl end of the growing chain. The growing protein exits the ribosome through the polypeptide exit tunnel in the large subunit. [10]
Elongation starts when the fMet-tRNA enters the P site, causing a conformational change which opens the A site for the new aminoacyl-tRNA to bind. This binding is facilitated by elongation factor-Tu (EF-Tu), a small GTPase. For fast and accurate recognition of the appropriate tRNA, the ribosome utilizes large conformational changes (conformational proofreading). [11] Now the P site contains the beginning of the peptide chain of the protein to be encoded and the A site has the next amino acid to be added to the peptide chain. The growing polypeptide connected to the tRNA in the P site is detached from the tRNA in the P site and a peptide bond is formed between the last amino acids of the polypeptide and the amino acid still attached to the tRNA in the A site. This process, known as peptide bond formation, is catalyzed by a ribozyme (the 23S ribosomal RNA in the 50S ribosomal subunit). [12] Now, the A site has the newly formed peptide, while the P site has an uncharged tRNA (tRNA with no amino acids). The newly formed peptide in the A site tRNA is known as dipeptide and the whole assembly is called dipeptidyl-tRNA. The tRNA in the P site minus the amino acid is known to be deacylated. In the final stage of elongation, called translocation, the deacylated tRNA (in the P site) and the dipeptidyl-tRNA (in the A site) along with its corresponding codons move to the E and P sites, respectively, and a new codon moves into the A site. This process is catalyzed by elongation factor G (EF-G). The deacylated tRNA at the E site is released from the ribosome during the next A-site occupation by an aminoacyl-tRNA again facilitated by EF-Tu. [13]
The ribosome continues to translate the remaining codons on the mRNA as more aminoacyl-tRNA bind to the A site, until the ribosome reaches a stop codon on mRNA(UAA, UGA, or UAG).
The translation machinery works relatively slowly compared to the enzyme systems that catalyze DNA replication. Proteins in bacteria are synthesized at a rate of only 18 amino acid residues per second, whereas bacterial replisomes synthesize DNA at a rate of 1000 nucleotides per second. This difference in rate reflects, in part, the difference between polymerizing four types of nucleotides to make nucleic acids and polymerizing 20 types of amino acids to make proteins. Testing and rejecting incorrect aminoacyl-tRNA molecules takes time and slows protein synthesis. In bacteria, translation initiation occurs as soon as the 5' end of an mRNA is synthesized, and translation and transcription are coupled. This is not possible in eukaryotes because transcription and translation are carried out in separate compartments of the cell (the nucleus and cytoplasm).
Termination occurs when one of the three termination codons moves into the A site. These codons are not recognized by any tRNAs. Instead, they are recognized by proteins called release factors, namely RF1 (recognizing the UAA and UAG stop codons) or RF2 (recognizing the UAA and UGA stop codons). These factors trigger the hydrolysis of the ester bond in peptidyl-tRNA and the release of the newly synthesized protein from the ribosome. A third release factor RF-3 catalyzes the release of RF-1 and RF-2 at the end of the termination process.
The post-termination complex formed by the end of the termination step consists of mRNA with the termination codon at the A-site, an uncharged tRNA in the P site, and the intact 70S ribosome. Ribosome recycling step is responsible for the disassembly of the post-termination ribosomal complex. [14] Once the nascent protein is released in termination, Ribosome Recycling Factor and Elongation Factor G (EF-G) function to release mRNA and tRNAs from ribosomes and dissociate the 70S ribosome into the 30S and 50S subunits. IF3 then replaces the deacylated tRNA releasing the mRNA. All translational components are now free for additional rounds of translation.
Depending on the tRNA, IF1–IF3 may also perform recycling. [15]
Translation is carried out by more than one ribosome simultaneously. Because of the relatively large size of ribosomes, they can only attach to sites on mRNA 35 nucleotides apart. The complex of one mRNA and a number of ribosomes is called a polysome or polyribosome. [16]
When bacterial cells run out of nutrients, they enter stationary phase and downregulate protein synthesis. Several processes mediate this transition. [17] For instance, in E. coli, 70S ribosomes form 90S dimers upon binding with a small 6.5 kDa protein, ribosome modulation factor RMF. [18] [19] These intermediate ribosome dimers can subsequently bind a hibernation promotion factor (the 10.8 kDa protein, HPF) molecule to form a mature 100S ribosomal particle, in which the dimerization interface is made by the two 30S subunits of the two participating ribosomes. [20] The ribosome dimers represent a hibernation state and are translationally inactive. [21] A third protein that can bind to ribosomes when E. coli cells enter the stationary phase is YfiA (previously known as RaiA). [22] HPF and YfiA are structurally similar, and both proteins can bind to the catalytic A- and P-sites of the ribosome. [23] [24] RMF blocks ribosome binding to mRNA by preventing interaction of the messenger with 16S rRNA. [25] When bound to the ribosomes the C-terminal tail of E. coli YfiA interferes with the binding of RMF, thus preventing dimerization and resulting in the formation of translationally inactive monomeric 70S ribosomes. [25] [26]
In addition to ribosome dimerization, the joining of the two ribosomal subunits can be blocked by RsfS (formerly called RsfA or YbeB). [27] RsfS binds to L14, a protein of the large ribosomal subunit, and thereby blocks joining of the small subunit to form a functional 70S ribosome, slowing down or blocking translation entirely. RsfS proteins are found in almost all eubacteria (but not archaea) and homologs are present in mitochondria and chloroplasts (where they are called C7orf30 and iojap, respectively). However, it is not known yet how the expression or activity of RsfS is regulated.
Another ribosome-dissociation factor in Escherichia coli is HflX, previously a GTPase of unknown function. Zhang et al. (2015) showed that HflX is a heat shock–induced ribosome-splitting factor capable of dissociating vacant as well as mRNA-associated ribosomes. The N-terminal effector domain of HflX binds to the peptidyl transferase center in a strikingly similar manner as that of the class I release factors and induces dramatic conformational changes in central intersubunit bridges, thus promoting subunit dissociation. Accordingly, loss of HflX results in an increase in stalled ribosomes upon heat shock and possibly other stress conditions. [28]
Several antibiotics exert their action by targeting the translation process in bacteria. They exploit the differences between prokaryotic and eukaryotic translation mechanisms to selectively inhibit protein synthesis in bacteria without affecting the host.
Ribosomes are macromolecular machines, found within all cells, that perform biological protein synthesis. Ribosomes link amino acids together in the order specified by the codons of messenger RNA molecules to form polypeptide chains. Ribosomes consist of two major components: the small and large ribosomal subunits. Each subunit consists of one or more ribosomal RNA molecules and many ribosomal proteins. The ribosomes and associated molecules are also known as the translational apparatus.
In biology, translation is the process in living cells in which proteins are produced using RNA molecules as templates. The generated protein is a sequence of amino acids. This sequence is determined by the sequence of nucleotides in the RNA. The nucleotides are considered three at a time. Each such triple results in addition of one specific amino acid to the protein being generated. The matching from nucleotide triple to amino acid is called the genetic code. The translation is performed by a large complex of functional RNA and proteins called ribosomes. The entire process is called gene expression.
Transfer RNA is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length. In a cell, it provides the physical link between the genetic code in messenger RNA (mRNA) and the amino acid sequence of proteins, carrying the correct sequence of amino acids to be combined by the protein-synthesizing machinery, the ribosome. Each three-nucleotide codon in mRNA is complemented by a three-nucleotide anticodon in tRNA. As such, tRNAs are a necessary component of translation, the biological synthesis of new proteins in accordance with the genetic code.
The Shine–Dalgarno (SD) sequence is a ribosomal binding site in bacterial and archaeal messenger RNA, generally located around 8 bases upstream of the start codon AUG. The RNA sequence helps recruit the ribosome to the messenger RNA (mRNA) to initiate protein synthesis by aligning the ribosome with the start codon. Once recruited, tRNA may add amino acids in sequence as dictated by the codons, moving downstream from the translational start site.
Eukaryotic translation is the biological process by which messenger RNA is translated into proteins in eukaryotes. It consists of four phases: initiation, elongation, termination, and recapping.
Aminoacyl-tRNA is tRNA to which its cognate amino acid is chemically bonded (charged). The aa-tRNA, along with particular elongation factors, deliver the amino acid to the ribosome for incorporation into the polypeptide chain that is being produced during translation.
In molecular biology, initiation factors are proteins that bind to the small subunit of the ribosome during the initiation of translation, a part of protein biosynthesis.
EF-Tu is a prokaryotic elongation factor responsible for catalyzing the binding of an aminoacyl-tRNA (aa-tRNA) to the ribosome. It is a G-protein, and facilitates the selection and binding of an aa-tRNA to the A-site of the ribosome. As a reflection of its crucial role in translation, EF-Tu is one of the most abundant and highly conserved proteins in prokaryotes. It is found in eukaryotic mitochondria as TUFM.
A bacterial initiation factor (IF) is a protein that stabilizes the initiation complex for polypeptide translation.
50S is the larger subunit of the 70S ribosome of prokaryotes, i.e. bacteria and archaea. It is the site of inhibition for antibiotics such as macrolides, chloramphenicol, clindamycin, and the pleuromutilins. It includes the 5S ribosomal RNA and 23S ribosomal RNA.
The prokaryotic small ribosomal subunit, or 30S subunit, is the smaller subunit of the 70S ribosome found in prokaryotes. It is a complex of the 16S ribosomal RNA (rRNA) and 19 proteins. This complex is implicated in the binding of transfer RNA to messenger RNA (mRNA). The small subunit is responsible for the binding and the reading of the mRNA during translation. The small subunit, both the rRNA and its proteins, complexes with the large 50S subunit to form the 70S prokaryotic ribosome in prokaryotic cells. This 70S ribosome is then used to translate mRNA into proteins.
EF-G is a prokaryotic elongation factor involved in mRNA translation. As a GTPase, EF-G catalyzes the movement (translocation) of transfer RNA (tRNA) and messenger RNA (mRNA) through the ribosome.
A protein synthesis inhibitor is a compound that stops or slows the growth or proliferation of cells by disrupting the processes that lead directly to the generation of new proteins.
Translational regulation refers to the control of the levels of protein synthesized from its mRNA. This regulation is vastly important to the cellular response to stressors, growth cues, and differentiation. In comparison to transcriptional regulation, it results in much more immediate cellular adjustment through direct regulation of protein concentration. The corresponding mechanisms are primarily targeted on the control of ribosome recruitment on the initiation codon, but can also involve modulation of peptide elongation, termination of protein synthesis, or ribosome biogenesis. While these general concepts are widely conserved, some of the finer details in this sort of regulation have been proven to differ between prokaryotic and eukaryotic organisms.
The P-site is the second binding site for tRNA in the ribosome. The other two sites are the A-site (aminoacyl), which is the first binding site in the ribosome, and the E-site (exit), the third. During protein translation, the P-site holds the tRNA which is linked to the growing polypeptide chain. When a stop codon is reached, the peptidyl-tRNA bond of the tRNA located in the P-site is cleaved releasing the newly synthesized protein. During the translocation step of the elongation phase, the mRNA is advanced by one codon, coupled to movement of the tRNAs from the ribosomal A to P and P to E sites, catalyzed by elongation factor EF-G.
EF-P is an essential protein that in bacteria stimulates the formation of the first peptide bonds in protein synthesis. Studies show that EF-P prevents ribosomes from stalling during the synthesis of proteins containing consecutive prolines. EF-P binds to a site located between the binding site for the peptidyl tRNA and the exiting tRNA. It spans both ribosomal subunits with its amino-terminal domain positioned adjacent to the aminoacyl acceptor stem and its carboxyl-terminal domain positioned next to the anticodon stem-loop of the P site-bound initiator tRNA. The EF-P protein shape and size is very similar to a tRNA and interacts with the ribosome via the exit “E” site on the 30S subunit and the peptidyl-transferase center (PTC) of the 50S subunit. EF-P is a translation aspect of an unknown function, therefore It probably functions indirectly by altering the affinity of the ribosome for aminoacyl-tRNA, thus increasing their reactivity as acceptors for peptidyl transferase.
In molecular biology, translation initiation factor IF-3 is one of the three factors required for the initiation of protein biosynthesis in bacteria. IF-3 is thought to function as a fidelity factor during the assembly of the ternary initiation complex which consists of the 30S ribosomal subunit, the initiator tRNA and the messenger RNA. IF-3 is a basic protein that binds to the 30S ribosomal subunit. The chloroplast homolog enhances the poly(A,U,G)-dependent binding of the initiator tRNA to its ribosomal 30s subunits. IF1–IF3 may also perform ribosome recycling.
In molecular biology, VAR1 protein domain, otherwise known as variant protein 1, is a ribosomal protein that forms part of the small ribosomal subunit in yeast mitochondria. Mitochondria possess their own ribosomes responsible for the synthesis of a small number of proteins encoded by the mitochondrial genome. VAR1 is the only protein in the yeast mitochondrial ribosome to be encoded in the mitochondria - the remaining approximately 80 ribosomal proteins are encoded in the nucleus. VAR1 along with 15S rRNA are necessary for the formation of mature 37S subunits.
Ribosomal L28e protein family is a family of evolutionarily related proteins. Members include 60S ribosomal protein L28.
Ribosomal pause refers to the queueing or stacking of ribosomes during translation of the nucleotide sequence of mRNA transcripts. These transcripts are decoded and converted into an amino acid sequence during protein synthesis by ribosomes. Due to the pause sites of some mRNA's, there is a disturbance caused in translation. Ribosomal pausing occurs in both eukaryotes and prokaryotes. A more severe pause is known as a ribosomal stall.