Why is the first nucleotide of a codon more enriched in ribo-seq reads?

Question

[Cacau]

An example is shown as follows (fig. a). The first nucleotide of a codon is always overrepresented than others.

Answer 1

[Chris Fields]

Seems like a test question, but I'll bite.

Just speculating wildly (would like to see a definitive answer with links, maybe you can post the paper link this came from?)

IIRC the ribosome pauses awaiting the next tRNA then ratchets forward a codon at a time. Maybe when they are captured for Ribo-Seq they are mostly in a paused state (e.g. mostly positioned relative to a codon). So maybe this is reflective of that iterative ratcheting process, and it's completely normal?

[WouterDeCoster]

I had the same in mind and vaguely remember reading this somewhere.