Beyond rRNA and snRNA: tRNA as a 2′-O-methylation target for nucleolar and Cajal body box C/D RNPs

Box C/D small nucleolar RNAs (snoRNAs) and small Cajal body (CB) RNAs (scaRNAs) form ribonucleoprotein (RNP) complexes to mediate 2′-O-methylation of rRNAs and small nuclear RNAs (snRNAs), respectively. The site of methylation is determined by antisense elements in the box C/D RNAs that are complementary to sequences in target RNAs. However, numerous box C/D RNAs in mammalian cells lack antisense elements to rRNAs or snRNAs; thus, their targets remain unknown. In this issue of Genes & Development , Vitali and Kiss (pp. 741–746) demonstrate that “orphan” nucleolar box C/D snoRNA SNORD97 and CB box C/D scaRNA SCARNA97 contain antisense elements that target the wobble cytidine at position 34 of human elongator tRNA Met (CAT) for 2′-O-methylation (C 34 m). C 34 m is jointly mediated by SNORD97 and SCARNA97 despite their apparently different intranuclear locations. Furthermore, the investigators demonstrate that C 34 m prohibits site-specific cleavage of tRNA Met (CAT) into tRNA fragments (tRFs) by the stress-responsive endoribonuclease angiogenin, thereby uncovering a role for SNORD97 and SCARNA97 in the biogenesis of tRFs, which modulate a diverse set of cellular functions in human health and disease.