Translation starts when a ribosome assembles on a messenger RNA (mRNA) molecule and recognizes a start codon, almost always AUG. This three-letter genetic signal tells the ribosome “begin here,” and it triggers the placement of the first amino acid of a new protein. But the full answer involves more than just a codon. A cascade of molecular events must happen before that first amino acid is set in place, and the process differs between bacteria and the cells of animals, plants, and fungi.
The Start Codon: AUG and Beyond
AUG is the near-universal start codon. In every domain of life, it’s the primary signal that tells the ribosome where to begin reading the mRNA and building a protein. In eukaryotic cells (human, animal, plant, and fungal cells), genes were long assumed to always begin at the first AUG on the mRNA. In bacteria, about 80% of annotated genes start with AUG.
The remaining 20% of bacterial genes use alternative start codons. Around 12% begin with GUG and about 8% with UUG, with rarer cases of AUU and AUC. Eukaryotes use non-AUG codons more often than previously thought: recent studies using drugs that freeze ribosomes at their start sites revealed that roughly 50% of all translation initiation events in eukaryotic cells occur at non-AUG codons, with CUG being the most common alternative (accounting for 15 to 16% of initiation sites). Even at these non-standard codons, the cell still places methionine as the first amino acid.
How Bacteria Start Translation
Bacterial ribosomes are recruited directly to the right spot on the mRNA by a short sequence called the Shine-Dalgarno sequence, discovered in 1974. This is a stretch of AG-rich nucleotides located just upstream of the start codon. It works by physically base-pairing with a complementary CU-rich region at the tail end of the small ribosomal subunit’s RNA (the 16S rRNA). This pairing anchors the ribosome in exactly the right position so the start codon lands in the ribosome’s active site, called the P-site.
Once positioned, a special initiator transfer RNA (tRNA) carrying the amino acid N-formylmethionine (fMet) pairs with the start codon. This is distinct from the methionine used by eukaryotes: bacteria chemically modify their starter methionine by adding a formyl group. With fMet-tRNA locked into the P-site and the start codon recognized, the large ribosomal subunit joins in, forming the complete 70S ribosome. The bacterium is now ready to read the mRNA and build the protein.
How Eukaryotic Cells Start Translation
Eukaryotic initiation is more elaborate, involving at least a dozen helper proteins called eukaryotic initiation factors (eIFs). The process begins with the small ribosomal subunit (the 40S subunit) loading up with initiation factors eIF3 and eIF1A. Meanwhile, a separate “ternary complex” forms: the initiator methionine-tRNA binds to the protein eIF2, which is itself bound to the energy molecule GTP. This ternary complex then joins the 40S subunit to create what’s known as the 43S pre-initiation complex, with eIF1A helping lock everything together.
This 43S complex is then recruited to the mRNA’s 5′ cap, a chemical tag at the very beginning of the message. From there, the complex scans along the mRNA, moving in one direction until it encounters the first AUG codon in a favorable sequence context. That context matters: the nucleotides immediately surrounding the AUG determine how efficiently the ribosome recognizes it. The key positions are the nucleotide three spots before the AUG (position -3) and the one immediately after the third letter of the codon (position +4). Having a purine at both of those positions, specifically an adenine at -3 and a guanine at +4, creates a “strong” start signal. Pyrimidines at those positions create a “weak” context that the ribosome may skip past. This surrounding pattern is called the Kozak sequence.
Once the scanning complex finds a strong AUG, the initiator tRNA pairs with it in the P-site. At this point, eIF2 burns its GTP (a reaction triggered by eIF5), which causes all the other initiation factors to fall away. This clears the way for the large 60S ribosomal subunit to join, forming the complete 80S ribosome. One more round of GTP burning by eIF5B releases this final factor, and the ribosome is now a fully functional protein-making machine, ready to begin adding amino acids one by one.
Cap-Independent Initiation Through IRES
Not all translation begins at the 5′ cap. Some mRNAs contain internal ribosome entry sites (IRES), which are complex RNA structures that recruit ribosomes directly to a spot in the middle of the message. This was first discovered in viruses, which use IRES elements to hijack the host cell’s ribosomes. But certain cellular mRNAs also use this strategy, particularly when the cell is under stress and normal cap-dependent translation shuts down.
IRES elements work by folding into elaborate three-dimensional shapes, including stem-loops, junctions, and structures called pseudoknots. These shapes act as molecular scaffolds that grab onto ribosomal subunits through direct RNA-to-RNA contact or with the help of RNA-binding proteins. Some viral IRES elements are so efficient they can assemble the ribosome at the start site without needing any initiation factors at all. Dicistroviruses, for example, have an IRES that folds into a structure mimicking a tRNA, which slots into the ribosome and lets it begin translating without the normal molecular helpers.
How Cells Control When Translation Starts
Because translation consumes enormous energy, cells tightly regulate the initiation step. One of the most powerful control switches targets eIF2, the protein that delivers the first methionine-tRNA to the ribosome. When a cell is stressed (by starvation, infection, or damage from reactive oxygen species), enzymes add a phosphate group to eIF2. This phosphorylated eIF2 grips tightly to eIF2B, the recycling factor that normally recharges eIF2 with fresh GTP. With eIF2B locked up, the cell can’t regenerate the ternary complex, and global protein production drops sharply.
This single modification can reduce overall translation across the entire cell while paradoxically allowing a small number of stress-response mRNAs to be translated more efficiently. It’s an elegant on/off switch: one phosphorylation event on one initiation factor throttles the production of thousands of proteins simultaneously.
Prokaryotic vs. Eukaryotic Initiation at a Glance
- Ribosome recruitment: Bacteria use Shine-Dalgarno base-pairing to place the ribosome directly at the start codon. Eukaryotes recruit the ribosome to the 5′ cap and scan forward to find AUG.
- First amino acid: Bacteria use N-formylmethionine (fMet). Eukaryotes use standard methionine.
- Ribosome size: Bacteria assemble a 70S ribosome from 30S and 50S subunits. Eukaryotes assemble an 80S ribosome from 40S and 60S subunits.
- Initiation factors: Bacteria need three initiation factors (IF1, IF2, IF3). Eukaryotes require at least twelve eIFs.
- Alternative start codons: Bacteria commonly use GUG and UUG for about 20% of genes. Eukaryotes use non-AUG codons more than previously recognized, with CUG being the most frequent alternative.