Determining the defining lengths between mature microRNAs/small interfering RNAs and tinyRNAs

MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are loaded into Argonaute (AGO) proteins, forming RNA-induced silencing complexes (RISCs). The assembly process establishes the seed, central, 3′ supplementary, and tail regions across the loaded guide, enabling the RISC to recognize target RNAs for silencing. This guide segmentation is caused by anchoring the 3′ end at the AGO PAZ domain, but the minimum guide length required for the conformation remains to be studied because the current miRNA size defined by Dicer processing is ambiguous. Using a 3′ → 5′ exonuclease ISG20, we determined the lengths of AGO-associated miR-20a and let-7a with 3′ ends that no longer reach the PAZ domain. Unexpectedly, miR-20a and let-7a needed different lengths, 19 and 20 nt, respectively, to maintain their RISC conformation. This difference can be explained by the low affinity of the PAZ domain for the adenosine at g19 of let-7a, suggesting that the tail-region sequence slightly alters the minimum guide length. We also present that 17-nt guides are sufficiently short enough to function as tinyRNAs (tyRNAs) whose 3′ ends are not anchored at the PAZ domain. Since tyRNAs do not have the prerequisite anchoring for the standardized guide segmentation, they would recognize targets differently from miRNAs and siRNAs.