The intricate process of human mtDNA replication requires the coordinated action of both transcription and replication machineries. Transcription and replication events at the lagging strand of mtDNA prompt the formation of a stem-loop structure (OriL) and the synthesis of a ∼25 nt RNA primer by mitochondrial RNA polymerase (mtRNAP). The mechanisms by which mtRNAP recognizes OriL, initiates transcription, and transfers the primer to the replisome are poorly understood. We found that transcription initiation at OriL involves slippage of the nascent transcript. The transcript slippage is essential for initiation complex stability and its ability to translocate the mitochondrial DNA polymerase gamma, PolG, which pre-binds to OriL, downstream of the replication origin thus allowing for the primer synthesis. Our data suggest the primosome assembly at OriL—a complex of mtRNAP and PolG—can efficiently generate the primer, transfer it to the replisome, and protect it from degradation by mitochondrial endonucleases.

中文翻译：

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线粒体复制起点 OriL 的转录起始和引物生成机制

人类 mtDNA 复制的复杂过程需要转录和复制机器的协调作用。mtDNA 滞后链上的转录和复制事件促进了茎环结构 (OriL) 的形成和线粒体 RNA 聚合酶 (mtRNAP) 合成约 25 nt RNA 引物。mtRNAP 识别 OriL、启动转录和将引物转移到复制体的机制知之甚少。我们发现 OriL 的转录起始涉及新生转录本的滑移。转录物滑移对于起始复合物的稳定性及其转移线粒体 DNA 聚合酶 γ、PolG 的能力至关重要，PolG 与复制起点下游的 OriL 预结合，从而允许引物合成。我们的数据表明原始体组装在OriL ——mtRNAP 和 PolG 的复合物——可以有效地产生引物，将其转移到复制体，并保护它免受线粒体内切酶的降解。