Leaky Scanning

During eukaryotic translation, ribosomes scan mRNA starting from the 5' end until they encounter the first AUG sequence, the start codon, and initiate protein synthesis.

However, an mRNA may have two or more AUG codons contained in its sequence. Ribosomes will sometimes fail to recognize the first AUG codon and instead start protein synthesis from a start codon further down the mRNA strand. 

This phenomenon is called leaky scanning, and it allows the production of multiple types of protein from the same mRNA.

A particular consensus sequence, known as the Kozak sequence, determines whether a ribosome will start protein synthesis at the first start codon or skip it. In this sequence, the A of the first start codon AUG is numbered as +1. The nucleotides after are positive, and the nucleotides before are negative.

The optimal sequence occurs when a purine is present at the -3 position and a guanine is present at the +4 position. In the -3 position, adenine is more effective than guanine at initiating translation.

Changes in the rest of the nucleotide sequence have little influence on protein synthesis.

When an optimal sequence is present, almost all the ribosomes will initiate protein