Putatively Noncoding Transcripts Show Extensive Association with Ribosomes

There have been recent surprising reports that whole genes can evolve de novo from noncoding sequences. This would be extraordinary if the noncoding sequences were random with respect to amino acid identity. However, if the noncoding sequences were previously translated at low rates, with the most strongly deleterious cryptic polypeptides purged by selection, then de novo gene origination would be more plausible. Here we analyze Saccharomyces cerevisiae data on noncoding transcripts found in association with ribosomes. We find many such transcripts. Although their average ribosomal densities are lower than those of protein-coding genes, a significant proportion of noncoding transcripts nevertheless have ribosomal densities comparable to those of coding genes. Most show increased ribosomal association in response to starvation, as has been previously reported for other noncoding sequences such as untranslated regions and introns. In rich media, ribosomal association is correlated with start codons but is not usually consistent and contiguous beyond that, suggesting that translation occurs only at low rates. One transcript contains a 28-codon open reading frame, which we name RDT1, which shows evidence of translation, and may be a new protein-coding gene that originated de novo from noncoding sequence. But the bulk of the ribosomal association cannot be attributed to unannotated protein-coding genes. Our primary finding of extensive ribosome association shows that a necessary precondition for selective purging is met, making de novo gene evolution more plausible. Our analysis is also proof of principle of the utility of ribosomal profiling data for the purpose of gene annotation.