tRNA, yRNA, and rRNA fragment excisions do not involve canonical microRNA biogenesis machinery.

The excision of specific tRNA-derived small RNAs (tsRNAs), yRNA-derived small RNAs (ysRNAs) and ribosomal RNA-derived small RNAs (rsRNAs) is now well established. Several reports have suggested many of these fragments function much like traditional microRNAs (miRNAs). That said, whereas the expressions of the majority of appreciably expressed miRNAs in HCT116 colon cancer cells are significantly decreased in individual knockouts (KOs) of DROSHA, DGCR8, XPO5, and DICER, on average, only 3.5% of tsRNA, ysRNA, and rsRNA expressions are impaired. Conversely, tsRNA, ysRNA, and rsRNA expressions are significantly increased in each of these KOs as compared to WT. As such, although DICER has been suggested to be involved with the expression of specific tsRNAs, ysRNAs, and rsRNAs, our study finds no evidence supporting the involvement of any of these canonical miRNA biogenesis enzymes in their expressions.