Determinants of selectivity in the dicing mechanism

The RNase III enzyme, DICER, is instrumental in the production of small RNAs, including miRNAs and siRNAs, by cleaving their precursors, such as pre-miRNAs, shRNAs, and long duplex RNAs. Utilizing High-throughput (HT) cleavage assays, our study delves into the cleavage activity of DICER. We challenge the widely accepted 2-nt loop counting rule in the previous study, revealing a divergent mechanism, the bipartite base pairing rule. This rule directs DICER's cleavage sites via the RNase III domain. Moreover, we demystify the recognition mechanism of the previously identified YCR motif. Building on this understanding, a secondary YCR motif that also influences DICER's cleavage sites has been discovered. We also address a long-debated issue concerning DICER's cleavage sites on long stem RNAs, such as pre-siRNAs or long shRNAs/pre-miRNAs. Our study shows that the dsRBD plays a crucial role in determining the cleavage sites of DICER in long-stem RNAs. In sum, our research provides a comprehensive understanding of several fundamental DICER mechanisms, challenging the long-standing model of the loop counting rule. This newfound knowledge reshapes our understanding of DICER's mechanisms, providing a robust foundation for future studies investigating the vast number of DICER mutations linked to various diseases. Overall design: We overexpressed different DICER proteins in human cells, including DICER-WT, DICER-delta-Helicase, DICER-delta-dsRBD. We then conducted in vitro assays using the purified DICER proteins and synthetic RNA substrates. The terms "Control" and "double cleavage" in the sample names refer to the original shRNA substrates and the RNA cleavage products by one of the DICER proteins, respectively. They are then cloned by adapter ligation, RT-PCR to obtain the libraries.