Ready to dice: In the biogenesis of small RNAs, the Dicer family endonucleases act as a molecular ruler to cut the substrate RNA into defined lengths. Wang et al . report a structure of the dicing-competent state of plant DICER LIKE PROTEIN 3 (DCL3) in complex with a pre–small interfering RNA (pre-siRNA). Toward one end of the pre-siRNA, DCL3 uses a positively charged pocket and an aromatic cap to specifically recognize the 5′-phosphorylated adenosine of the guide strand and the 3′ overhang of the complementary strand, respectively. On the other end, the paired ribonuclease III domains of DCL3 cut both strands of the RNA, determining the precise length of the product small RNA. —DJ

5-Methylcytosine (m ⁵ C) is a RNA modification that exists in tRNAs and rRNAs and was recently found in mRNAs. Although it has been suggested to regulate diverse biological functions, whether m ⁵ C RNA modification influences adult stem cell development remains undetermined. In this study, we show that Ypsilon schachtel (YPS), a homolog of human Y box binding protein 1 (YBX1), promotes germ line stem cell (GSC) maintenance, proliferation, and differentiation in the Drosophila ovary by preferentially binding to m ⁵ C-containing RNAs. YPS is genetically demonstrated to function intrinsically for GSC maintenance, proliferation, and progeny differentiation in the Drosophila ovary, and human YBX1 can functionally replace YPS to support normal GSC development. Highly conserved cold-shock domains (CSDs) of YPS and YBX1 preferentially bind to m ⁵ C RNA in vitro. Moreover, YPS also preferentially binds to m ⁵ C-containing RNAs, including mRNAs, in germ cells. The crystal structure of the YBX1 CSD-RNA complex reveals that both hydrophobic stacking and hydrogen bonds are critical for m ⁵ C binding. Overexpression of RNA-binding–defective YPS and YBX1 proteins disrupts GSC development. Taken together, our findings show that m ⁵ C RNA modification plays an important role in adult stem cell development.

Dual recognition of H3K4me3 and DNA by the CRA-type ISWI complex subunit ARID5 is required for gene expression, development, and the floral transition.