Leaky scanning is a technique commonly employed by viruses to express multiple proteins from a single sequence of RNA. When RNA is translated in eukaryotes, the 40s ribosomal RNA (40s rRNA) will scan from the 5' end of the RNA until it reaches the first AUG codon where it will begin translation from that open reading frame. This is problematic for intracellular viruses that need to express proteins from alternate open reading frames, so they utilise leaky scanning to overcome this. Leaky scanning occurs when the AUG start codon is not surrounded by an optimal sequence of nucleotides, known as a Kozak consensus sequence, causing this start codon to be occasionally skipped (the frequency of this occurring depends on the particular surrounding sequence). This causes the 40s rRNA to continue to the next start codon - which may or may not be out of frame where it will form the full ribosomal complex and begin synthesising a protein.
The second open reading frame usually overlaps the first open reading frame, but this is not always the case.
This technique gives some control over the level of gene expression, as the genes on the alternate open reading frame are translated less frequently.
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