Lindahl L, Archer R H, Zengel J M
Department of Biology, University of Rochester, NY 14627.
Nucleic Acids Res. 1994 Dec 11;22(24):5399-407. doi: 10.1093/nar/22.24.5399.
We have extended the system of Nogi et al. (Proc. Natl. Acad. Sci. USA 88, 1991, 3962-3966) for transcription of rRNA from an RNA polymerase II promoter in strains lacking functional RNA polymerase I. In our strains two differentially marked rRNA transcription units can be expressed alternately. Using this system we have shown that the A2 processing site in the internal transcribed spacer 1 (ITS1) of the pre-rRNA is dispensable. According to the accepted processing scheme, the A2 site serves to separate the parts of the primary rRNA transcript that are destined for incorporation into the two ribosomal subunits. However, we have found that, when A2 is impaired, separation of the small and large subunit rRNAs occurs at a processing site further downstream in ITS1, indicating that alternate pathways for ITS1 processing exist. Short deletions in the A2 region still allow residual processing at the A2 site. Mapping of the cleavage sites in such deletion transcripts suggests that sequences downstream of the A2 site are used for determining the position of the cleavage.
我们扩展了野木等人（《美国国家科学院院刊》88卷，1991年，3962 - 3966页）的系统，用于在缺乏功能性RNA聚合酶I的菌株中从RNA聚合酶II启动子转录rRNA。在我们构建的菌株中，两个差异标记的rRNA转录单元能够交替表达。利用该系统我们发现，前体rRNA内部转录间隔区1（ITS1）中的A2加工位点是可有可无的。根据公认的加工方案，A2位点用于分隔初级rRNA转录本中注定要整合到两个核糖体亚基中的部分。然而，我们发现，当A2位点功能受损时，小亚基和大亚基rRNA的分离发生在ITS1中更下游的一个加工位点，这表明存在ITS1加工的替代途径。A2区域的短缺失仍允许在A2位点进行残余加工。对此类缺失转录本中切割位点的定位表明，A2位点下游的序列用于确定切割位置。
