While near-cognate codons are frequently used for translation initiation in eukaryotes, their efficiencies are usually low (<10% compared to an AUG in optimal context). Here, we describe a rare case of highly efficient near-cognate initiation. A CUG triplet located in the 5′ leader of POLG messenger RNA (mRNA) initiates almost as efficiently (∼60 to 70%) as an AUG in optimal context. This CUG directs translation of a conserved 260-triplet-long overlapping open reading frame (ORF), which we call POLGARF (POLG Alternative Reading Frame). Translation of a short upstream ORF 5′ of this CUG governs the ratio between POLG (the catalytic subunit of mitochondrial DNA polymerase) and POLGARF synthesized from a single POLG mRNA. Functional investigation of POLGARF suggests a role in extracellular signaling. While unprocessed POLGARF localizes to the nucleoli together with its interacting partner C1QBP, serum stimulation results in rapid cleavage and secretion of a POLGARF C-terminal fragment. Phylogenetic analysis shows that POLGARF evolved ∼160 million y ago due to a mammalian-wide interspersed repeat (MIR) transposition into the 5′ leader sequence of the mammalian POLG gene, which became fixed in placental mammals. This discovery of POLGARF unveils a previously undescribed mechanism of de novo protein-coding gene evolution.

虽然近同源密码子经常用于真核生物中的翻译起始，但它们的效率通常很低（与最佳上下文中的 AUG 相比<10%）。在这里，我们描述了一个罕见的高效近同源启动案例。位于POLG信使 RNA (mRNA) 5' 前导的 CUG 三联体在最佳环境中的启动效率几乎与 AUG 一样有效 (~60% 至 70%)。该 CUG 指导保守的 260 个三联体长重叠开放阅读框 (ORF) 的翻译，我们称之为POLGARF（POLG替代阅读框）。该 CUG 的上游短 ORF 5' 的翻译决定了 POLG（线粒体 DNA 聚合酶的催化亚基）和由单个POLG合成的 POLGARF 之间的比率mRNA。POLGARF 的功能研究表明在细胞外信号传导中的作用。虽然未加工的 POLGARF 与其相互作用的伙伴 C1QBP 一起定位于核仁，但血清刺激导致 POLGARF C 末端片段的快速切割和分泌。系统发育分析表明，POLGARF在大约1.6 亿年前进化，这是由于哺乳动物范围内的散在重复 (MIR) 转座到哺乳动物POLG基因的 5' 前导序列中，该前导序列在胎盘哺乳动物中变得固定。POLGARF的这一发现揭示了以前未描述的蛋白质编码基因从头进化的机制。