An octopus edits its RNA, especially in the nervous system

Cephalopods recode RNA transcripts at tens of thousands of sites, heavily in the nervous system, far beyond what a vertebrate does. The genome says one thing. The message that is read often says another.

Adenosine-to-inosine editing is a chemical change in an RNA transcript. The cell’s ribosome reads inosine as guanosine, so a protein can come out different from the gene that was transcribed. Vertebrates do this at a modest number of sites. In 2017 a comparison across cephalopods found recoding at an enormous scale in squid, octopus, and cuttlefish — tens of thousands of sites, enriched in proteins of the nervous system — and almost none of that scale in the nautilus or in other molluscs. The editing is concentrated where a nerve has to work, and a large fraction of the edits change the amino acid.

The same study noticed the cost. Genes that are heavily edited accumulate mutations more slowly, as if the genome is constrained by the need to keep the editing sites intact. Flexibility was gained in the transcript and given up in the gene. What most of those amino-acid changes do for the animal is not known, one protein at a time. Temperature-sensitive edits in a potassium channel are a worked example, not a theory of the whole set. The scale is the fact. A second nervous system, already in this library because so much of it is in the arms, is also a nervous system that rewrites its own messages after the DNA has spoken.