Initiation of Translation

The start codon position determines the reading frame for all downstream codons. If translation begins one nucleotide before or after the AUG start codon, every subsequent codon is misread, producing a non-functional amino acid sequence. Correct positioning ensures accurate protein synthesis from the beginning of translation.

The initiator tRNA carries methionine (or formylmethionine in bacteria) and contains conserved nucleotides recognized by eukaryotic initiation factors. It binds the P site of the small ribosomal subunit with eIF2 and GTP, forming the pre-initiation complex. This specialized tRNA is distinct from elongator tRNAs and ensures proper translation start.

The pre-initiation complex binds mRNA through eIF4E recognition of the 5' cap and eIF4G binding to poly(A) tail-binding proteins. Powered by ATP hydrolysis, the complex scans from 5' to 3' direction, with the initiator tRNA anticodon searching for the first AUG sequence. Upon codon-anticodon recognition, GTP hydrolysis allows the large ribosomal subunit to join.

Leaky scanning occurs when the pre-initiation complex skips the first AUG triplet if surrounding nucleotides differ substantially from the consensus sequence (5'-ACCAUGG-3'). The complex continues scanning to the next AUG. This mechanism allows cells and viruses to produce multiple proteins from a single mRNA molecule.

Bacterial mRNAs lack 5' caps and instead contain a Shine-Dalgarno sequence upstream of the first AUG codon. This consensus AGGAGGU sequence base pairs with complementary 16S rRNA on the small ribosomal subunit, serving as the ribosomal binding site. Bacterial ribosomes can assemble directly on start codons, allowing polycistronic mRNAs to encode multiple proteins.

Once the large ribosomal subunit joins and forms an intact ribosome, a new tRNA carrying the second amino acid can bind to the A-site. This allows protein synthesis to begin with the addition of the second amino acid to the growing polypeptide chain, transitioning from initiation to elongation.

The cell uses initial recognition of the 5' cap by eIF4E to verify proper mRNA processing before translation begins. This quality control mechanism ensures that only mature, capped mRNAs are translated. Most eukaryotic mRNAs are monocistronic, encoding a single protein, which further ensures translation fidelity.