- Prokaryotic translation
Prokaryotic translation is the process by which
messenger RNAis translated into proteins in prokaryotes.
Initiation of translation in prokaryotes involves the assembly of the components of the translation system which are: the two ribosomal subunits (small and large), the
mRNAto be translated, the first (formyl) aminoacyl tRNA (the tRNA charged with the first amino acid), GTP (as a source of energy), and three initiation factors (IF1, IF2 and IF3) which help the assembly of the initiation complex.
ribosomeconsists of three 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, fMet-tRNAfMet, 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.
Initiation of translation begins with the 50S and 30S ribosomal subunits dissociated. IF1 (initiation factor 1) blocks the A site to insure that the fMet-tRNA can bind only to the P site and that no other aminoacyl-tRNA can bind in the A site during initiation, while IF3 blocks the E site and prevents the two subunits from associating. IF2 is a small
GTPasewhich binds fmet-tRNAfMet and helps its binding with the small ribosomal subunit. The 3' end of the 16S rRNA of the small 30S ribosomal subunit recognizes the ribosomal binding site on the mRNA( Shine-Dalgarno sequenceor SD), through its anti-SD sequence, 5-10 base pairs upstream of the start codon. The Shine-Dalgarno sequence is found only in prokaryotes. This helps to correctly position the ribosome onto the mRNA so that the P site is directly on the AUG initiation codon. IF3 helps to position fMet-tRNAfMet into the P site, such that fMet-tRNAfMet interacts via base pairing with the mRNA initiation codon (AUG). Initiation ends as the large ribosomal subunit joins the complex causing the dissociation of initiation factors. Note that prokaryotes can differentiate between a normal AUG (coding for methionine) and an AUG initiation codon (coding for formyl-methionineand indicating the start of a new translation process).
Elongation of the
polypeptidechain involves addition of amino acids to the carboxylend of the growing chain. The growing proteinexits the ribosomethrough the polypeptide exit tunnel in the large subunit [Structure fo the "E. coli" protein-coducting channel bound to at translating ribosome, K. Mitra, et al. "Nature" (2005), vol 438, p 318] .
Elongation starts when the fmet-tRNA enters the P site, causing a
conformational changewhich opens the A site for the new aminoacyl-tRNA to bind. This binding is facilitated by elongation factor-Tu (EF-Tu), a small GTPase. 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 bondis 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, peptidyltransferase, an activity intrinsic to the 23S ribosomal RNAin the 50S ribosomal subunit. Now, the A site has newly formed peptide, while the P site has an unloaded tRNA (tRNA with no amino acids). In the final stage of elongation, "translocation", the ribosome moves 3 nucleotides towards the 3'end of mRNA. Since tRNAs are linked to mRNA by codon-anticodon base-pairing, tRNAs move relative to the ribosome taking the nascent polypeptide from the A site to the P site and moving the uncharged tRNA to the E exit site. This process is catalyzed by elongation factor G (EF-G).
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).
Termination occurs when one of the three
termination codonsmoves 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 hydrolysisof the esterbond 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. [Hirokawa et al. (2006) "The Ribosome Recycling Step: Consensus or Controversy?". Trends in Biochemical Sciences Vol. 31(3), 143-149.] Once the nascent protein is released in termination,
Ribosome Recycling Factorand 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.
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
Effect of antibiotics
antibioticsexert their action by targeting the translation process in bacteria. They exploit the differences between prokaryotic and eukaryotic translationmechanisms to selectively inhibit protein synthesis in bacteria without affecting the host. Examples include:
Puromycinhas a structure similar to the tyrosinyl aminoacyl-tRNA. Thus, it binds to the ribosomal A site and participates in peptide bond formation, producing peptidyl-puromycin. However, it does not engage in translocation and quickly dissociates from the ribosome causing a premature termination of polypeptide synthesis.
Streptomycincauses misreading of the genetic codein bacteria at relatively low concentrations and inhibits initiation at higher concentrations, by binding to the 30s ribosomal subunit.
aminoglycosidesas Tobramycinand Kanamycinprevent ribosomal association at the end of initiation step and cause misreading of the genetic code.
Tetracyclinesblock the A site on the ribosome, preventing the binding of aminoacyl tRNAs.
Chloramphenicolblocks the peptidyl transfer step of elongation on the 50s ribosomal subunit in both bacteria and mitochondria.
Macrolidesand Lincosamidesbind to the 50s ribosomal subunits inhibiting the peptidyltransferase reaction or translocation or both.
Prokaryotic initiation factors
Prokaryotic elongation factors
Prokaryotic release factors
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