Figure 4: Secondary structures of archaeal guide sRNAs. (a) The secondary structure of box C/D sRNA. The box C/D and the box C′/D′ motifs form the kink-turn and kink-loop motif, respectively. The two unpaired strands of sequences flanked by the box C/D and box C′/D′ motifs base-pair with substrate and are called guide sequences. (b) The secondary structure of a single-hairpin box H/ACA sRNA. The 3′ end of the stem is immediately followed by a box H or box ACA motif. The pseuduouridylation pocket is in the middle of the hairpin with two unpaired strands of sequences that are capable of substrate base-pairing.