Inferring targeting modes of Argonaute-loaded tRNA fragments.

Transfer RNA fragments (tRFs) are an emerging class of small RNA molecules derived from mature or precursor tRNAs. They are found across a wide range of organisms and tissues, in small RNA fraction or loaded to Argonaute in numbers comparable to microRNAs. Their functions and mechanisms of action are largely unknown, and results obtained on individual tRFs are often hard to generalize. Here we predicted binding mechanisms and specific target interaction sites of 26 human Argonaute-loaded tRFs of different types using large-scale meta-analyses of available experimental data. Strikingly, our findings matched all interaction sites detected in a recent experimental screen, confirming the validity of our computational approach. Such sites are primarily located on the 5' end of tRFs and often involve additional binding along the tRF length, similar to microRNAs. Indicative of multiple layers of regulation, diverse regulatory non-coding RNAs comprised a third of the tRF targets, with the rest being protein-coding transcripts. In the latter, coding sequence and 3' UTRs were the likely primary target regions, although we observed interactions of tRFs with 5' UTRs. Another novel phenomenon we report, a large number of putative interactions between tRFs and introns, is compatible with the roles of Argonaute in the nucleus. Further, observed tRF-intron binding modes suggest a mechanism of interaction of tRFs with Argonaute-dependent introns, and we predict here >20 candidate introns of this type. Taken together, these results present tRFs as regulatory molecules with a rich functional spectrum.