Background: Spliceosomal snRNAs and ribosomal RNAs in metazoans contain numerous modified residues that are functionally important. The most common modifications are site-specific 2'-O-methylation and pseudouridylation, both directed by small ribonucleoprotein particles. Each particle is composed of a short guide RNA and a set of several proteins. All previously characterized modification guide RNAs in metazoa are encoded in and processed from introns.
Results: We have identified and characterized three novel guide RNAs for conserved 2'-O-methylation of U2, U4, and U12 snRNAs. Two guides, termed mgU2-25/61 and mgU12-22/U4-8, appear to be independently transcribed as judged by the presence of methylated guanosine caps at their 5' ends and upstream promoters similar to those of telomerase RNA. These guide RNAs are each composed of a canonical box C/D snoRNA and a novel box C/D snoRNA-like domain, where the C'/D' motif, rather than C/D, can be folded into a conserved kink-turn structure. The snoRNA-like domains are predicted to direct 2'-O-methylation of invariant G residues that occupy analogous positions in the U2 and U12 snRNA secondary structures. A third guide, mgU2-19/30 RNA, is composed of two canonical box C/D snoRNA domains encoded within a single intron.
Conclusions: This is the first description in metazoan cells of 5'-capped modification guide RNAs that appear to be independently transcribed. Since plant, yeast, and protozoan guide RNAs are mostly independently transcribed, the identification of such RNAs argues that ancestral metazoans possessed independently transcribed guide RNAs and only later, during the evolution of metazoan organisms, did the guide RNA genes shift to introns.