The relatively recent focus on the widespread occurrence and abundance of circular RNAs (circRNA) in the human cell nucleus has sparked an intensive interest in their existence and possible roles in cell gene expression and physiology. The presence of circRNAs in mammalian mitochondria, however, has been under-explored. Mitochondrial mRNAs differ from those produced from nuclear genes because they lack introns and are transcribed as poly-cistronic transcripts that are endonucleolytically cleaved, leaving transcripts with very small 5' and 3' UTRs. Circular RNAs have been identified in the semi-autonomous organelles of single-celled organisms and plants but their purpose has not been clearly demonstrated. The goal of our project was to test the hypothesis, processed mRNAs are circularized in vertebrate mitochondria as a necessary RNA processing step prior to translation. Mitochondrial mRNAs were isolated from the human cell line HEK293 and evidence of circularization sought by treating RNA with RNAse-R and then amplifying putative 3'-5' junction sites. Sequence results demonstrated the occurrence of mRNA circularization within each coding region of the mitochondrial genome. However, in most cases the circRNAs carried coding regions that had been truncated, suggesting they were not translatable. Quantification of the circularized versions of the mRNAs revealed they comprise a small portion (∼10%) of the total mRNA. These findings demonstrate that mRNA circularization occurs in mammalian mitochondria but it does not appear to play a role in making translatable mRNAs.

中文翻译：

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线粒体 mRNA 片段在人 HEK 细胞系中被环化

最近对人类细胞核中环状 RNA (circRNA) 的广泛存在和丰度的关注引发了人们对其存在以及在细胞基因表达和生理学中可能发挥的作用的浓厚兴趣。然而，哺乳动物线粒体中 circRNA 的存在尚未得到充分探索。线粒体 mRNA 与核基因产生的 mRNA 不同，因为它们缺乏内含子，并且转录为多顺反子转录物，这些转录物被核酸内切酶切割，留下非常小的 5' 和 3' UTR。在单细胞生物和植物的半自主细胞器中已经鉴定出环状 RNA，但它们的用途尚未得到明确证实。我们项目的目标是检验假设，加工过的 mRNA 在脊椎动物线粒体中被环化，作为翻译前必要的 RNA 加工步骤。从人类细胞系 HEK293 中分离出线粒体 mRNA，并通过用 RNAse-R 处理 RNA，然后放大推定的 3'-5' 连接位点来寻找环化的证据。序列结果证明在线粒体基因组的每个编码区域内发生了 mRNA 环化。然而，在大多数情况下，circRNA 带有被截短的编码区，表明它们不可翻译。mRNA 环化版本的定量显示它们占总 mRNA 的一小部分（约 10%）。这些发现表明 mRNA 环化发生在哺乳动物线粒体中，但它似乎没有在产生可翻译的 mRNA 中发挥作用。