Figure 1

Models accounting for the intronic location of snoRNAs. A) The introns-first model proposes that the intronic location of snoRNAs dates back to the origin of genetically encoded protein synthesis [17]. In this model, transcription of RNA genes requires subsequent processing of precursor RNAs to release functional RNAs. Processing via splicing leads to the production of processed snoRNAs, with intervening material spliced together. This material is later coopted for stabilisation of tRNA interactions during peptide synthesis, leading to the emergence of the first mRNAs. In this model, the first proteins serve to augment RNA function, so the association between snoRNAs and ribosomal protein-coding genes is historical and evolutionarily stable. B) Intronic organisation of snoRNAs is the result of selection for coexpression of snoRNA and its host gene [18]. This model does not explicitly state what the ancestral condition would have been, and is therefore compatible with this organisation being ancestral (panel A), a more recent de novo origin of the snoRNA, or with this association being the result of snoRNA retrotransposition.