During translation initiation, the ribosome recruits an mRNA and selects the start codon of the Open Reading Frame (ORF)
m RNAS of prokaryotes have an extended 5′ untranslated region (5′UTR) and an (SD) located 8–10 nt upstream of the start codon (usually AUG).
Interactions between the SD sequence and the complementary anti-SD (aSD) sequence in 16S ribosomal RNA (rRNA) recruits the small 30 S ribosomal subunit and anchors the 30S ribosomal subunit at the correct location on the mRNA template.
Initiation is promoted by initiation factors IF1, IF2, and IF3. IF1 enhances the activities of IF2 and IF3.IF2 is a GTPase that recruits the initiator fMet-tRNAfMet. IF3 interferes with subunit association. It also ensures the fidelity of fMet-tRNAfMet selection over the elongator aminoacyl-tRNAs (aa-tRNAs), and helps to discriminate against mRNAs with unfavorable translation initiation regions (TIRs)
The initiator tRNA then interacts with the start codon AUG (or rarely, GUG). This tRNA carries the amino acid methionine, which is formylated after its attachment to the tRNA and thus the Charged t RNA , fMet-tRNAMetf is formed . This is mediated by the initiation factor IF-2.
The initiating fMet is usually removed after translation is complete.
After the formation of the initiation complex, the 30S ribosomal subunit is joined by the 50S subunit to form the 70S translation complex.
Elongation
Elongation includes repetitive cycles of decoding the m RNA , peptide bond formation, and translocation.
Elongation begins as soon as the second codon of the ORF becomes accessible for reading by the next amino acylated tRNAs
Elongation is facilitated by translation factors EF-Tu, EF-G ,EF-Ts EF-P and EF-4.
The second codon in the P site is recognized by aminoacylated -tRNAs. A ternary complex of aa-tRANA ,EF-Tu and GTP is attached to the P site .This interaction triggers GTP hydrolysis by EF-Tu. After Pi release, EF-Tu rearranges into the guanosine diphosphate (GDP)-bound form and releases the aa-tRNA.
The amino group of the amino acid attached to the A-site tRNA makes a peptide bond with the carboxyl group of the amino acid attached to the P-site tRNA. This reaction is catalyzed by peptidyl transferase, an RNA-based ribozyme that is integrated into the 50S ribosomal subunit. Now the A site carries a peptide comprising of two amino acids and leaves the uncharged t RNA in the p site .
During elongation the ribosomes move one codon along the mRNA in a process called Translocation. Translocation is promoted by EF-G at the cost of GTP hydrolysis .During each translocation event, a new charged tRNAs enter at the A site, then shift to the P site, and then finally to the E site for removal. Ribosomal movements are induced by conformational changes that advance the ribosome by three bases in the 3′ direction.
This process of elongation continues until the ribosome arrives at the stop codon.
Amazingly, the E. coli translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200 amino-acid protein can be translated in just 10 seconds.
Termination
Termination occurs when the ribosome encounters a stop codon UAG/UAA and UGA/UAA, respectively in the mRNA.
These stop codons are recognized by the termination (or release) factors RF1 and RF2.
Another termination factor, RF3, facilitates turnover of RF1 and RF2 but is not required for peptidyl-tRNA hydrolysis.
The mechanism of termination encompasses three steps:
recognition of the stop codon,
hydrolysis of the ester bond of the peptidyl-tRNA (these two steps are accomplished by RF1 or RF2) and
dissociation of RF1/RF2 with the help of RF3.
The releasing factors instruct peptidyl transferase to add a water molecule to the carboxyl end of the P-site amino acid. This reaction forces the P-site amino acid to detach from its tRNA, and releases the newly made protein.
The small and large ribosomal subunits dissociate from the mRNA and from each other; they are recruited almost immediately into another translation initiation complex. After many ribosomes have completed translation, the mRNA is degraded so the nucleotides can be reused in another transcription reaction.