Strength of the Translation Initiation Signal Sequence of mRNA as Studied by the Quantification Method: Effect of Nucleotides at Positions from +4 to +6 upon the Recognition of ATG and Alternative Initiation Codons

Concerning the translation initiation signal in vertebrate mRNAs, although a consensus sequence, (GCC)GCC(A or G)CCATGG, has been proposed, the actual initiation sequences differ from it to a greater or lesser degree. Kozak monitored selection by ribosomes of the first versus second ATG codons as a function of introducing mutations at positions +4, +5 and +6 of the first ATG codon. Six different flanking codons, starting with G, strongly enhanced the selection of the first ATG codon. Except for T at position +5, ATG codon recognition was unaffected by most mutations at positions +5 and +6. These experimental results were analyzed using a quantification method proposed previously, and a 18-nucleotide sequence including ATG and nucleotides in positions +4, +5 and +6 was characterized by its sample score (strength of translation initiation signal). The experimental results were well understood based on the strength of the signal.