Life without tRNAArg–adenosine deaminase TadA: evolutionary consequences of decoding the four CGN codons as arginine in Mycoplasmas and other Mollicutes
Life without tRNAArg–adenosine deaminase TadA: evolutionary consequences of decoding the four CGN codons as arginine in Mycoplasmas and other Mollicutes
复制标题
没有 tRNAArg-腺苷脱氨酶 TadA 的生命：在支原体和其他软体动物中将四个 CGN 密码子解码为精氨酸的进化后果
作者：
S. Yokobori;A. Kitamura;H. Grosjean;Y. Bessho
在大多数细菌中，两个tRNA解码四个精氨酸CGN密码子。一个携带摆动肌苷的tRNA（tRNAArgICG）读取CGU、CGC和CGA密码子，而另一个携带摆动胞苷的tRNA（tRNAArgCCG）读取剩余的CGG密码子。支原体和其他柔膜菌的简化基因组缺少编码tRNAArgCCG的基因。这就提出了这些生物如何解码CGG密码子的问题。检查36个柔膜菌基因组的基因编码tRNAArg和TadA酶，负责摆动肌苷的形成，提出了一个进化的情况下，tadA基因突变首先发生。这使得能够阅读所有CGN密码子的非脱氨基tRNAArgACG暂时积累。这一假设在含有一小部分tRNAArgACG和非脱氨基摆动腺苷的山羊支原体中得到了验证。柔膜菌的亚群通过失去突变的tRNAArgCCG和tadA继续进化，然后获得新的tRNAArgUCG。这允许进一步tRNAArgACG突变与tRNAArgGCG或其消失，留下一个单一的tRNAArgUCG来解码四个CGN密码子。我们的模型的关键点是，A到I的脱氨活性必须在tadA基因丢失之前得到控制，从而允许柔膜菌逐步进化到另一种解码策略。
In most bacteria, two tRNAs decode the four arginine CGN codons. One tRNA harboring a wobble inosine (tRNAArgICG) reads the CGU, CGC and CGA codons, whereas a second tRNA harboring a wobble cytidine (tRNAArgCCG) reads the remaining CGG codon. The reduced genomes of Mycoplasmas and other Mollicutes lack the gene encoding tRNAArgCCG. This raises the question of how these organisms decode CGG codons. Examination of 36 Mollicute genomes for genes encoding tRNAArg and the TadA enzyme, responsible for wobble inosine formation, suggested an evolutionary scenario where tadA gene mutations first occurred. This allowed the temporary accumulation of non-deaminated tRNAArgACG, capable of reading all CGN codons. This hypothesis was verified in Mycoplasma capricolum, which contains a small fraction of tRNAArgACG with a non-deaminated wobble adenosine. Subsets of Mollicutes continued to evolve by losing both the mutated tRNAArgCCG and tadA, and then acquired a new tRNAArgUCG. This permitted further tRNAArgACG mutations with tRNAArgGCG or its disappearance, leaving a single tRNAArgUCG to decode the four CGN codons. The key point of our model is that the A-to-I deamination activity had to be controlled before the loss of the tadA gene, allowing the stepwise evolution of Mollicutes toward an alternative decoding strategy.
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