Savino R, Gerbi S A
Division of Biology and Medicine, Brown University, Providence, RI 02912.
Biochimie. 1991 Jun;73(6):805-12. doi: 10.1016/0300-9084(91)90060-e.
Recently it has been reported that U3 snRNA is necessary for: (a) internal cleavage at +651/+657 within the external transcribed spacer (ETS) of mouse precursor ribosomal RNA (pre-rRNA); and (b) cleavage at the 5' end of 5.8S rRNA in Xenopus oocytes. To study if U3 snRNA plays a role at more than one processing site in the same system, we have investigated whether internal cleavage sites exist within the ETS of Xenopus oocyte pre-rRNA. The ETS of Xenopus pre-rRNA contains the consensus sequence for the mammalian early processing site (+651/+657 in mouse pre-rRNA), but freshly prepared RNA from Xenopus oocytes has no cuts in this region. The only putative cleavage sites we found in the ETS of Xenopus oocyte pre-rRNA are a cluster further downstream of the mouse early processing site consensus sequence. This cluster is not homologous to the mouse +651/+657 sites because unlike the latter it is (a) not abolished by disruption of U3 snRNA, (b) not cleaved during early steps of pre-rRNA processing, and (c) lacks sequence similarity to the +651/+657 consensus. Therefore, pre-rRNA of Xenopus oocytes does not cleave within the ETS as an early step in rRNA processing. We conclude that cleavage within the ETS is not an obligatory early step needed for the rest of rRNA maturation.
最近有报道称，U3小核仁RNA（snRNA）对于以下过程是必需的：（a）小鼠前体核糖体RNA（pre-rRNA）外部转录间隔区（ETS）内+651/+657处的内部切割；以及（b）非洲爪蟾卵母细胞中5.8S rRNA 5'端的切割。为了研究U3 snRNA在同一系统中是否在多个加工位点发挥作用，我们研究了非洲爪蟾卵母细胞pre-rRNA的ETS内是否存在内部切割位点。非洲爪蟾pre-rRNA的ETS包含哺乳动物早期加工位点的共有序列（小鼠pre-rRNA中的+651/+657），但从非洲爪蟾卵母细胞新鲜制备的RNA在该区域没有切割。我们在非洲爪蟾卵母细胞pre-rRNA的ETS中发现的唯一假定切割位点是位于小鼠早期加工位点共有序列下游更远的一个簇。这个簇与小鼠的+651/+657位点不同源，因为与后者不同的是，它（a）不会因U3 snRNA的破坏而被消除，（b）在pre-rRNA加工的早期步骤中不会被切割，并且（c）与+651/+657共有序列缺乏序列相似性。因此，非洲爪蟾卵母细胞的pre-rRNA在rRNA加工的早期步骤中不会在ETS内切割。我们得出结论，ETS内的切割不是rRNA其余成熟过程所需的必经早期步骤。
