Conserved spacing between the box C/D and C'/D' RNPs of the archaeal box C/D sRNP complex is required for efficient 2'-O-methylation of target RNAs.

RNA-guided nucleotide modification complexes direct the post-transcriptional nucleotide modification of both archaeal and eukaryotic RNAs. We have previously demonstrated that efficient 2'-O-methylation activity guided by an in vitro reconstituted archaeal box C/D sRNP requires juxtaposed box C/D and C'/D' RNP complexes. In these experiments, we investigate the importance of spatially positioning the box C/D and C'/D' RNPs within the sRNP complex for nucleotide modification. Initial sequence analysis of 245 archaeal box C/D sRNAs from both Eukyarchaeota and Crenarchaeota kingdoms revealed highly conserved spacing between the box C/D and C'/D' RNA motifs. Distances between boxes C to D' and C' to D (D' and D spacers, respectively) exhibit highly constrained lengths of 12 nucleotides (nt). Methanocaldococcus jannaschii sR8 sRNA, a model box C/D sRNA with D and D' spacers of 12 nt, was mutated to alter the distance between the two RNA motifs. sRNAs with longer or shorter spacer regions could still form sRNPs by associating with box C/D core proteins, L7, Nop56/58, and fibrillarin, comparable to wild-type sR8. However, these reconstituted box C/D sRNP complexes were severely deficient in methylation activity. Alteration of the D and D' spacer lengths disrupted the guided methylation activity of both the box C/D and C'/D' RNP complexes. When only one spacer region was altered, methylation activity of the corresponding RNP was lost. Collectively, these results demonstrate the importance of box C/D and C'/D' RNP positioning for preservation of critical inter-RNP interactions required for efficient box C/D sRNP-guided nucleotide methylation.