RNA PolII is involved in ribosomal RNA transcriptio

Currently, the accepted mechanism of production of 18S, 5.8S and 25S ribosomal RNA components in yeast, is polycistronic transcription by RNA Polymerase I, followed by processing. We show here that Candida albicans, during nutritional limitations produces some of these molecules in a second way, leaving them resistant to a 5’-phosphate requiring exonuclease digestion, as they have more than one 5’ phosphate. This suggested new transcription. Applying Pol I and Pol II inhibitors BMH-21 and DRB, anti-RNA cap antibody, EIF4F subunit of cap-binding protein, chromatin immune-precipitation and nucleolar immune fluorescent microscopy, we show them to be products of Pol II. We also show that rapamycin inhibition of TOR also leads to their production. Oligo ligation/primer extension shows their 5’ ends to be at the processing sites. This appears to be a system to maintain protein production during nutritional limitation. The mechanism allowing Pol II to produce them remains to be elucidated.目前学界公认的酵母18S、5.8S及25S核糖体RNA（ribosomal RNA, rRNA）组分的生物合成机制为：经由RNA聚合酶I（RNA Polymerase I, Pol I）完成多顺反子转录，随后进行加工修饰。本研究发现，白色念珠菌（Candida albicans）在营养限制条件下可通过第二条途径生成部分此类RNA分子：由于这类分子携带多个5’端磷酸基团，因此可抵御依赖5’端磷酸的核酸外切酶消化。这提示存在此前未被报道的全新转录途径。本研究通过使用Pol I与Pol II抑制剂BMH-21、DRB，抗RNA帽抗体，帽结合蛋白EIF4F亚基，染色质免疫沉淀实验以及核仁免疫荧光显微镜成像技术，证实此类RNA分子实为Pol II的转录产物。此外，本研究还发现雷帕霉素对TOR（Target of Rapamycin, TOR）通路的抑制作用同样可诱导此类分子的生成。寡核苷酸连接/引物延伸实验结果显示，此类分子的5’端恰好位于已知的RNA加工位点处。该机制或为真菌在营养限制条件下维持蛋白质合成提供了一套代偿调控通路。而关于Pol II能够介导此类分子合成的具体分子机制，仍有待进一步阐明。