Bacterial ribosomal RNAs are synthesized by a dedicated, conserved transcription-elongation complex that transcribes at high rates, shields RNA polymerase from premature termination, and supports co-transcriptional RNA folding, modification, processing, and ribosomal subunit assembly by presently unknown mechanisms. We have determined cryo-electron microscopy structures of complete Escherichia coli ribosomal RNA transcription elongation complexes, comprising RNA polymerase; DNA; RNA bearing an N-utilization-site-like anti-termination element; Nus factors A, B, E, and G; inositol mono-phosphatase SuhB; and ribosomal protein S4. Our structures and structure-informed functional analyses show that fast transcription and anti-termination involve suppression of NusA-stabilized pausing, enhancement of NusG-mediated anti-backtracking, sequestration of the NusG C-terminal domain from termination factor ρ, and the ρ blockade. Strikingly, the factors form a composite RNA chaperone around the RNA polymerase RNA-exit tunnel, which supports co-transcriptional RNA folding and annealing of distal RNA regions. Our work reveals a polymerase/chaperone machine required for biosynthesis of functional ribosomes.

细菌核糖体RNA是由专用的，保守的转录-延伸复合体合成的，该复合体以高速率转录，保护RNA聚合酶免于过早终止，并通过目前未知的机制支持共转录RNA的折叠，修饰，加工和核糖体亚基组装。我们已经确定了完整大肠杆菌的冷冻电子显微镜结构核糖体RNA转录延伸复合物，包含RNA聚合酶；脱氧核糖核酸; RNA，带有类似N-利用位点的抗终止元件；Nus因子A，B，E和G；肌醇单磷酸酶SuhB; 和核糖体蛋白S4。我们的结构和结构相关的功能分析表明，快速转录和抗终止涉及抑制NusA稳定的暂停，增强NusG介导的抗回溯，从终止因子ρ隔离NusG C末端结构域和ρ封锁。令人惊讶的是，这些因素在RNA聚合酶RNA出口通道周围形成了复合RNA分子伴侣，从而支持共转录RNA折叠和远端RNA区域的退火。我们的工作揭示了功能性核糖体生物合成所需的聚合酶/分子伴侣机器。