Bernhardt Harold S
Department of Biochemistry, University of Otago, Dunedin 9016, New Zealand.
Life (Basel). 2016 Feb 24;6(1):10. doi: 10.3390/life6010010.
We have previously proposed that tRNA(Gly) was the first tRNA and glycine was the first amino acid incorporated into the genetic code. The next two amino acids incorporated would have been the other two small hydrophilic amino acids serine and aspartic acid, which occurred through the duplication of the tRNA(Gly) sequence, followed by mutation of its anticodon by single C to U transition mutations, possibly through spontaneous deamination. Interestingly, however, tRNA(Ser) has a different structure than most other tRNAs, possessing a long variable arm; because of this tRNA(Ser) is classified as a class II tRNA. Also, serine codons are found not only in the bottom right-hand corner of the genetic code table next to those for glycine and aspartic acid, but also in the top row of the table, next to those for two of the most hydrophobic amino acids, leucine and phenylalanine. In the following, I propose that the class II tRNA structure of tRNA(Ser) and the arrangement of serine codons in the genetic code provide clues to the early evolution of tRNA and the genetic code. In addition, I address Di Giulio's recent criticism of our proposal that tRNA(Gly) was the first tRNA, and discuss how early peptides produced from a restricted amino acid alphabet of glycine, serine and aspartic acid might have possessed proteolytic activity, which is possibly important for the early recycling of amino acid monomers.
我们之前曾提出，tRNA（甘氨酸）是首个tRNA，甘氨酸是首个被纳入遗传密码的氨基酸。接下来被纳入的两个氨基酸会是另外两个小的亲水性氨基酸——丝氨酸和天冬氨酸，这是通过tRNA（甘氨酸）序列的复制发生的，随后其反密码子通过单个C到U的转换突变而发生突变，可能是通过自发脱氨作用。然而，有趣的是，tRNA（丝氨酸）与大多数其他tRNA具有不同的结构，拥有一个长可变臂；因此tRNA（丝氨酸）被归类为II类tRNA。此外，丝氨酸密码子不仅出现在遗传密码表右下角甘氨酸和天冬氨酸密码子旁边，还出现在该表的第一行，紧邻两个最疏水的氨基酸——亮氨酸和苯丙氨酸的密码子。在下面的内容中，我提出tRNA（丝氨酸）的II类tRNA结构以及遗传密码中丝氨酸密码子的排列为tRNA和遗传密码的早期进化提供了线索。此外，我回应了迪朱利奥最近对我们提出的tRNA（甘氨酸）是首个tRNA这一观点的批评，并讨论了由甘氨酸、丝氨酸和天冬氨酸组成的有限氨基酸字母表产生的早期肽如何可能具有蛋白水解活性，这可能对氨基酸单体的早期循环利用很重要。
