What is a Kozak sequence and its significance?

• A. A consensus sequence surrounding the start codon in eukaryotes that enhances initiation at AUG.
• B. A tRNA anticodon loop for recognizing start codon.
• C. A bacterial promoter recognized by sigma factor.
• D. A splicing signal in introns.

Answer: (A)

The Kozak sequence is a recognizable context around the start codon in eukaryotic mRNAs that makes the ribosome more likely to initiate translation at AUG. During initiation, the ribosome binds the 5' cap and scans for the start codon, and a favorable surrounding sequence helps the initiation factors correctly position AUG, increasing the efficiency of start-site selection. In vertebrates, the optimal pattern is GCCRCCaugG, with a purine at the -3 position and a G at +4, though exact matches vary among species. A strong Kozak context boosts translation efficiency and protein yield, while a weaker context can lead to leaky scanning, causing initiation at downstream AUGs or lower overall translation. This concept is distinct from a tRNA anticodon loop, which is involved in decoding codons during elongation; it is not a bacterial promoter recognized by sigma factors, which governs transcription initiation; and it is not a splicing signal in introns, which marks exon–intron boundaries. The Kozak sequence is a key factor in how effectively eukaryotic mRNAs are translated.