Bacteria translation is the process by which messenger RNA is translated into proteins in bacteria.
Contents |
Initiation of translation in bacteria involves the assembly of the components of the translation system which are: the two ribosomal subunits (50S & 30S subunits), the mRNA to 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.[1]
The ribosome has 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.
The selection of an initiation site (Usually an AUG codon) depends on the interaction between the 30S RNA and the mRNA template. The 30S subunit binds to the mRNA template at a purine rich region (the Shine-Dalgarno sequence) upstream of the AUG initiation codon. The Shine Dalgarno sequence is complementary to a pyrimidine rich region on the 16S rRNA component on the 30S subunit. During the formation of the initiation complex, these complementary nucleotide sequences pair to form a double stranded RNA structure that binds the mRNA to the ribosome in such a way that the initiation codon is placed at the P site.
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.[2]
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. 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). Now, the A site has the newly formed peptide, while the P site has an uncharged tRNA (tRNA with no amino acids). In the final stage of elongation, called translocation, the ribosome moves 3 nucleotides toward the 3' end of the mRNA. Since tRNAs are linked to mRNA by codon-anticodon base-pairing, the 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 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.[3] 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.
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.
Several antibiotics exert their action by targeting the translation process in bacteria. They exploit the differences between bacterial, archaeal and eukaryotic translation mechanisms to selectively inhibit protein synthesis in bacteria without affecting the host.
|
||||||||||||||
|
||||||||||||||||||||||||||||||||||||||