Identification of NLPY DIMT1 as catalytically inactive.A, crystal structure of wildtype DIMT1 (Protein Data Bank: 1ZQ9). Four amino acids (Asn128Leu129Pro130Tyr131) were shown directly contacting SAM. B, LC–MS/MS quantification of m26,6A levels in an unmodified RNA probe after in vitro methylation reaction with wildtype or NLPY DIMT1 variant. SAM was supplemented in all the in vitro methylation activity assays. C, TBE–urea gel purification of small RNAs extracted from wildtype and NPLY DIMT1 HEK 293T cells. D, LC–MS/MS quantification of RNAs extracted from the annotated region in (C). E, cell proliferation assays performed in DIMT1+/− + empty vector, DIMT1+/− + wildtype (wt), DIMT1+/− + E85A, and DIMT1+/− + NLPY cells. p Values were determined using a two-tailed Student's t test for unpaired samples. Error bars represent mean ± SD. ∗∗p < 0.01. DIMT1, dimethyladenosine transferase 1; HEK 293T, human embryonic kidney 293T cells.