Homo sapiens C/D box snoRNA U20. This snoRNA was originally characterized in an intron of the human nucleolin gene by Nicoloso et al. (1994). U20 was postulated to guide the 2'O-ribose methylation of 18S rRNA U1804 by Kiss-Laszlo et al. (1996). Cavaillé et al. (1996) showed that U20 mutants in which the natural 18S rRNA complementarity was substituted by a complementarity to an 18S or 28S rRNA region devoid of 5'O-ribose methylation could efficiently guide that modification at the predicted site. U20, the C/D box snoRNA U82 and the H/ACA box snoRNA U23 share the same host gene (nucleolin).
In vertebrates, U20 generally resides in the penultimate intron of the NCL gene. In zebrafish, a highly mutated copy resides in an upstream intron (see alignment), but no functional U20 snoRNA sequence could be found. The length of the 18S rRNA anti-sense sequence appears to have increased during evolution (Maden, 1986; Hedges et al., 1990; Quezada et al., 2006). Also note that U20 in fishes is devoid of D’ and C’ boxes.