Baumann U, Oro J
Department of Biochemistry, University of Houston, Texas.
Biosystems. 1993;29(2-3):133-41. doi: 10.1016/0303-2647(93)90089-u.
A diversification of the genetic code based on the number of codons available for the proteinous amino acids is established. Three groups of amino acids during evolution of the code are distinguished. On the basis of their chemical complexity those amino acids emerging later in a translation process are derived. Codon number and chemical complexity indicate that His, Phe, Tyr, Cys and either Lys or Asn were introduced in the second stage, whereas the number of codons alone gives evidence that Trp and Met were introduced in the third stage. The amino acids of stage 1 use purine-rich codons, while all the amino acids introduced in the second stage, in contrast, use pyrimidines in the third position of their codons. A low abundance of pyrimidines during early translation is derived. This assumption is supported by experiments on non-enzymatic replication and interactions of hairpin loops with a complementary strand. A back extrapolation concludes a high purine content of the first nucleic acids, which gradually decreased during their evolution. Amino acids independently available from prebiotic synthesis were thus correlated to purine-rich codons. Implications on the prebiotic replication are discussed also in the light of recent codon usage data.
基于可用于蛋白质氨基酸的密码子数量，建立了遗传密码的多样化。在密码子进化过程中区分出三组氨基酸。根据它们的化学复杂性，推导出在翻译过程中较晚出现的那些氨基酸。密码子数量和化学复杂性表明，组氨酸、苯丙氨酸、酪氨酸、半胱氨酸以及赖氨酸或天冬酰胺是在第二阶段引入的，而仅密码子数量就表明色氨酸和甲硫氨酸是在第三阶段引入的。第一阶段的氨基酸使用富含嘌呤的密码子，而相比之下，在第二阶段引入的所有氨基酸在其密码子的第三位使用嘧啶。由此推导出早期翻译过程中嘧啶的丰度较低。这一假设得到了非酶复制以及发夹环与互补链相互作用实验的支持。反向推断得出第一批核酸的嘌呤含量很高，在其进化过程中逐渐降低。因此，可从益生元合成中独立获得的氨基酸与富含嘌呤的密码子相关。还根据最近的密码子使用数据讨论了对益生元复制的影响。
