Modification of U6 spliceosomal RNA is guided by other small RNAs

Abstract

The vertebrate spliceosomal snRNAs are highly modi-fied by pseudouridylation and 2′-O-methylation. We have identified novel conserved small RNAs that can direct addition of two methyl groups in U6 snRNA, at A47 and C77. These guide RNAs, mgU6-47 (methylation guide for U6 snRNA residue 47) and mgU6-77 contain boxes C, C′, D, and D′ and associate with fibrillarin. Each RNA can form a duplex with U6 snRNA positioning A47 and C77 for 2′-O-methylation. The antisense element of mgU6-77 can also position C2970 of 28S rRNA for 2′-O-methylation. Depletion of mgU6-77 from Xenopus oocytes prevents 2′-O-methylation of both C77 in U6 and C2970 in 28S; methylation can be restored by injecting in vitro transcribed mgU6-77. Thus, mgU6-77 appears to function in the 2′-O-methylation of two distinct classes of cellular RNA, snRNA, and rRNA.