Nicoloso M, Qu L H, Michot B, Bachellerie J P
Laboratoire de Biologie Moléculaire Eucaryote du C.N.R.S., Université Paul-Sabatier, Toulouse, France.
J Mol Biol. 1996 Jul 12;260(2):178-95. doi: 10.1006/jmbi.1996.0391.
A growing number of small nucleolar RNAs (snoRNAs) are intron-encoded, contain the characteristic box C (UGAUGA) and box D (CUGA) motifs and exhibit long complementarities to conserved sequences in mature rRNAs. We have identified nine additional members of this family, U32 to U40. All but one are encoded in introns of ribosomal protein genes in vertebrates: U32 to U35 in rpL13a, U36 in rpL7a and U38 to U40 in rpS8. By contrast, U37 is encoded in elongation factor 2 gene. Interestingly, U32 and U36 each contain two complementarities (one to 18 S and the other to 28 S rRNA). U32 to U40 are fibrillarin-associated, devoid of a 5'-trimethyl-cap and display an exclusively nucleolar localization. They are all metabolically stable and roughly as abundant as previously reported members of this family. Characterization of their homologs in distant species shows that their 10 to 14 nt long rRNA complementarities are conserved. A clue on the function of this snoRNA family is provided by the comparative analysis of the largely expanded collection of their conserved duplexes with rRNA. Not only does each duplex span at least one site of 2'-O-ribose methylation in the rRNA but the modification site is always at the same position in the duplex, paired to the fifth nucleotide upstream from a box D motif in the snoRNA. Consistent with the notion that each snoRNA of this family guides one particular methylation along the rRNA sequence, we have detected several pairs of snoRNAs with overlapping complementarities to rRNA tracts with vicinal sites of ribose methylations. In each case, the two overlapping complementarities are shifted from each other by a distance equal to the spacing between the methylated sites which are thus found at the same position within each of the mutually exclusive duplexes. Finally, we have also identified, within three previously known snoRNAs, novel antisense elements able to form a canonical duplex around ribose-methylated sites in rRNA, which further supports the conclusion that the duplex structure provides the 2'-O-methyltransferase with the appropriate site-specificity on the substrate.
越来越多的小核仁RNA（snoRNA）由内含子编码，含有特征性的C盒（UGAUGA）和D盒（CUGA）基序，并与成熟rRNA中的保守序列表现出长互补性。我们又鉴定出了该家族的九个成员，即U32至U40。除一个成员外，其余所有成员均由脊椎动物核糖体蛋白基因的内含子编码：rpL13a中的U32至U35，rpL7a中的U36以及rpS8中的U38至U40。相比之下，U37由延伸因子2基因编码。有趣的是，U32和U36各自包含两个互补性（一个与18S互补，另一个与28S rRNA互补）。U32至U40与纤维蛋白相关，缺乏5'-三甲基帽，并且仅定位于核仁。它们在代谢上都很稳定，丰度与该家族先前报道的成员大致相同。对其在远缘物种中的同源物的表征表明，它们10至14个核苷酸长的rRNA互补性是保守的。通过对其与rRNA的大量扩展的保守双链体集合进行比较分析，为这个snoRNA家族的功能提供了线索。不仅每个双链体跨越rRNA中至少一个2'-O-核糖甲基化位点，而且修饰位点始终在双链体中的相同位置，与snoRNA中D盒基序上游的第五个核苷酸配对。与该家族的每个snoRNA沿着rRNA序列指导一种特定甲基化的观点一致，我们检测到几对snoRNA与具有相邻核糖甲基化位点的rRNA片段具有重叠的互补性。在每种情况下，两个重叠的互补性彼此偏移的距离等于甲基化位点之间的间距，因此在每个互斥双链体中的相同位置发现甲基化位点。最后，我们还在三个先前已知的snoRNA中鉴定出了新的反义元件，它们能够在rRNA中的核糖甲基化位点周围形成典型的双链体，这进一步支持了双链体结构为2'-O-甲基转移酶在底物上提供适当位点特异性的结论。
