RS and RGG repeats as primitive proteins at the transition between the RNA and RNP worlds

For many experimental biologists in the field of nuclear cell biology, low complexity repeats in nuclear proteins constitute a nuisance. They are difficult to express, impossible to crystalize and have low but near ubiquitous unwanted affinities toward many biomolecules. Examples of such nuclear protein repeats are RS (Arg-Ser) repeats in splicing factors, RGG (Arg-Gly-Gly) repeats in hnRNP proteins and FG (Phe-Gly) repeats in nuclear pore components. Here, I would like to present a more positive perspective for at least a subset of repeats and suggest that they are excellent candidates to have constituted the first proteins emerging from an RNA world.