rRNA modification in the nucleus and cytosol

Human ribosomal RNAs (rRNAs) contain about 200 residues that are enzymatically modified after transcription in the nucleolus (Maden and Khan 1977, Maden 1988, Maden and Hughes 1997, reviewed in Hernandez-Verdun et al. 2010, Boschi-Muller and Motorin 2013). The modified residues occur in regions of the rRNAs that are located in functionally important parts of the ribosome, notably in the A and P peptidyl transfer sites, the polypeptide exit tunnel, and intersubunit contacts (Polikanov et al. 2015, reviewed in Decatur and Fournier 2002, Chow et al. 2007, Sharma and Lafontaine 2015). The two most common modifications are pseudouridines and 2'-O-methylribonucleotides. Formation of pseudouridine from encoded uridine is catalyzed by box H/ACA small nucleolar ribonucleoprotein (snoRNP) complexes (reviewed in Hamma and Ferre-D'Amare 2010, Watkins and Bohnsack 2011, Ge and Yu 2013, Kierzek et al. 2014, Yu and Meier 2014) and methylation of the hydroxyl group of the 2' carbon is catalyzed by box C/D snoRNPs (Kiss-Laszlo et al. 1996, Lapinaite et al. 2013, reviewed in Watkins and Bohnsack 2011). The snoRNP complexes contain common sets of protein subunits and unique snoRNAs that guide each complex to its target nucleotide of the rRNA by base-pairing between the snoRNA and the rRNA (reviewed in Henras et al. 2004, Watkins and Bohnsack 2011). Other modifications of rRNA include 5-methylcytidine (reviewed in Squires and Preiss 2010), 1-methylpseudouridine, 7-methylguanosine, 6-dimethyladenosine, and 4-acetylcytidine (reviewed in Sharma and Lafontaine 2015). In yeast most modifications are introduced co-transcriptionally (Kos and Tollervey 2010, reviewed in Turowski and Tollervey 2015), however the order of modification events and pre-rRNA cleavage events is not well characterized.

人类核糖体RNA（rRNA）包含约200个残基，这些残基在转录后于核仁中经过酶促修饰（Maden和Khan 1977，Maden 1988，Maden和Hughes 1997，详见Hernandez-Verdun等人2010年的综述，Boschi-Muller和Motorin 2013）。这些修饰的残基位于rRNA的功能性重要区域，尤其是在核糖体的A和P肽基转移位点、多肽出口通道以及亚基间接触处（Polikanov等人2015年，参见Decatur和Fournier 2002年的综述，Chow等人2007年，Sharma和Lafontaine 2015年）。其中最常见的两种修饰为假尿苷和2'-O-甲基核糖核苷。编码尿苷转化为假尿苷的过程由H/ACA盒小核仁RNA核糖核蛋白（snoRNP）复合体催化（详见Hamma和Ferre-D'Amare 2010年的综述，Watkins和Bohnsack 2011年，Ge和Yu 2013年，Kierzek等人2014年，Yu和Meier 2014年）。2'碳羟基的甲基化则由C/D盒snoRNP催化（Kiss-Laszlo等人1996年，Lapinaite等人2013年，详见Watkins和Bohnsack 2011年的综述）。snoRNP复合体含有共同的蛋白质亚基集合和独特的snoRNA，通过snoRNA与rRNA之间的碱基配对引导每个复合体到达其目标核苷酸（详见Henras等人2004年的综述，Watkins和Bohnsack 2011年）。rRNA的其他修饰还包括5-甲基胞嘧啶（详见Squires和Preiss 2010年的综述），1-甲基假尿苷，7-甲基鸟苷，6-二甲基腺苷和4-乙酰胞嘧啶（详见Sharma和Lafontaine 2015年的综述）。在酵母中，大多数修饰是在转录过程中引入的（Kos和Tollervey 2010年，详见Turowski和Tollervey 2015年的综述），然而修饰事件和前体rRNA切割事件的顺序尚未得到充分表征。