Translation in prokaryotes is a highly coordinated, energy-driven process involving mRNA, tRNA, ribosome, and many accessory factors. The Central Dogma of Molecular Biology explains the flow of genetic information as:
DNA → RNA → Protein
Once a gene is transcribed into messenger RNA (mRNA), the next critical step is translation. Translation is the process by which the nucleotide sequence of mRNA is decoded into a specific sequence of amino acids, resulting in the formation of a polypeptide (protein). This makes translation the final and functional step of the central dogma, where genetic information is expressed as proteins that carry out vital cellular functions.
Translation in prokaryotes: Definition
It is the process by which the genetic information encoded in messenger RNA (mRNA) is converted into a specific sequence of amino acids, forming a polypeptide chain or protein. The process begins at the ribosome, the molecular machine of protein synthesis, and involves three key players: mRNA, transfer RNA (tRNA), and ribosomes.
Translation Location:
In prokaryotes, translation takes place in the cytoplasm on 70S ribosomes and is tightly coupled with transcription. As soon as an mRNA strand emerges from RNA polymerase, ribosomes immediately attach to it and begin building proteins. This simultaneous process allows bacteria to respond quickly to environmental changes, conserve energy, and ensure efficient growth. Thus, translation in prokaryotes is a fast, energy-driven, and highly regulated process that is essential for their survival.
Main Players in Translation:
- mRNA → carries genetic code (codons).
- tRNA → delivers amino acids via anticodon–codon pairing.
- Amino acids → building blocks of proteins.
- Ribosome (70S) → site of protein synthesis.
- Protein factors (IFs, EFs, RFs) → assist initiation, elongation, and termination.
Structure of mRNA in Prokaryotes
Prokaryotic mRNA has three important regions:
- 5′ UTR (Leader sequence):
- Contains the Shine–Dalgarno (SD) sequence.
- SD sequence is complementary to the 3′ end of 16S rRNA (30S subunit).
- Guides ribosome to the correct start site.
- Reading Frame:
- Starts with start codon (AUG → fMet).
- Contains codons (triplets) that specify amino acids.
- Ends with one of three stop codons (UAA, UAG, UGA).
- 3′ UTR (Trailer sequence):
- Follows the stop codon.
- No coding function, but helps in regulation and stability.
🧾 Transfer RNA (tRNA)
- Each tRNA has:
- Anticodon loop → complementary to mRNA codon.
- 3′ CCA end → amino acid attachment site.
- Initiator tRNA in prokaryotes:
- Carries formyl-methionine (fMet).
- Anticodon complementary to AUG.
- Example: Codon AUG → Anticodon UAC → tRNA brings fMet.
Ribosome – The Protein Factory
- Prokaryotic ribosome = 70S
- 30S subunit: contains 16S rRNA (decoding center).
- 50S subunit: contains 23S rRNA (peptidyl transferase activity) + 5S rRNA.
- Together form the active 70S ribosome.
- tRNA binding sites:
- A site (Aminoacyl site): entry site for new aminoacyl-tRNA.
- P site (Peptidyl site): holds growing peptide chain.
- E site (Exit site): releases empty/unloaded tRNA.
- Order: E → P → A
Phases of Translation in Prokaryotes
Like replication and transcription, translation occurs in three phases. Translation involves the coordinated action of mRNA, transfer RNA (tRNA), ribosomes, and protein factors through three main stages: initiation, elongation, and termination. The process begins when the ribosome recognizes the Shine–Dalgarno sequence on mRNA, followed by the binding of the initiator tRNA carrying formyl-methionine (fMet) to the start codon (AUG). Elongation then proceeds with the sequential addition of amino acids, until a stop codon signals termination.
1️⃣ Initiation
Goal: Assemble ribosome + mRNA + initiator tRNA at start codon.
Translation in prokaryotes starts when the ribosome recognizes a special sequence on the mRNA called the Shine-Dalgarno sequence, located just before the start codon AUG. The initiator tRNA, carrying a modified methionine (formyl-methionine or fMet), binds to this start codon, marking the beginning of protein synthesis.
Steps:
- Formation of pre-initiation complex:
- IF-1, IF-2 (with GTP), and IF-3 bind to 30S subunit.
- Prevents premature joining of 50S.
- mRNA and initiator tRNA binding:
- 30S binds to Shine–Dalgarno sequence via 16S rRNA.
- Initiator tRNA (fMet-tRNA^fMet) binds to start codon (AUG) at the P site.
- Formation of 70S initiation complex:
- 50S subunit joins.
- GTP hydrolysis releases initiation factors.
- Complete 70S initiation complex is formed → ready for elongation.
👉 Key Points for Exams:
- Start codon = AUG (codes for fMet).
- Initiator tRNA enters P site directly.
- Shine–Dalgarno pairs with 16S rRNA (30S).
2️⃣ Elongation
Goal: Add amino acids one by one to form a polypeptide chain.
From here, elongation proceeds as amino acids are added one by one, until a stop codon signals termination.
Steps:
- tRNA Entry (EF-Tu + GTP):
- Correct aminoacyl-tRNA enters the A site.
- EF-Tu hydrolyzes GTP only if codon–anticodon pairing is correct → proofreading.
- EF-Ts recycles EF-Tu.
- Peptide Bond Formation (Peptidyl Transferase):
- Catalyzed by 23S rRNA (ribozyme) of 50S.
- Growing peptide chain shifts from tRNA in P site → amino acid in A site.
- Translocation (EF-G + GTP):
- Ribosome shifts one codon forward.
- tRNA from P → E site (exit).
- tRNA from A → P site (peptide chain attached).
- A site becomes empty → ready for next aminoacyl-tRNA.
👉 Cycle repeats until stop codon is reached.
3️⃣ Termination
Goal: Stop protein synthesis and release polypeptide.
Process:
- Stop codon (UAA, UAG, UGA) enters A site.
- No tRNA matches these codons.
- Release factors (RFs):
- RF-1 → recognizes UAA, UAG.
- RF-2 → recognizes UAA, UGA.
- RF-3 → helps dissociate RF-1/RF-2 (GTP-dependent).
- RFs hydrolyze bond between polypeptide and tRNA in P site.
- Polypeptide chain is released.
- Ribosome dissociates into 30S and 50S.
Summary of Translation Factors
- Initiation: IF-1, IF-2 (GTP), IF-3.
- Elongation: EF-Tu (GTP), EF-Ts, EF-G (GTP).
- Termination: RF-1, RF-2, RF-3 (GTP).
Key Exam Insights
- Ribosome binding site: Shine–Dalgarno sequence (5′ UTR).
- Initiator tRNA: fMet-tRNA^fMet enters P site.
- Peptidyl transferase activity: 23S rRNA (50S subunit).
- Decoding center: 16S rRNA (30S subunit).
- Translocation factor: EF-G + GTP.
- Proofreading factor: EF-Tu + GTP.
- Stop codon recognition: RF-1 (UAA, UAG), RF-2 (UAA, UGA).
Final Takeaway
Translation in prokaryotes is a highly coordinated, energy-driven process involving mRNA, tRNA, ribosome, and many accessory factors.
The ability of prokaryotes to carry out transcription and translation simultaneously is a unique feature that maximizes efficiency.
As soon as an mRNA strand emerges from RNA polymerase, ribosomes immediately attach to it and start building proteins.
This tight coupling highlights why translation in prokaryotes is not only fast but also highly regulated.