Jukes T H, Osawa S, Muto A, Lehman N
Space Sciences Laboratory, University of California, Berkeley 94720.
Cold Spring Harb Symp Quant Biol. 1987;52:769-76. doi: 10.1101/sqb.1987.052.01.086.
Clues to evolution of the genetic code can be found by comparing usage of anticodons in various organisms and organelles. GC content of DNA varies, as a result of directional mutation pressure (AT/GC pressure), especially in bacteria. Low GC in Mycoplasma is accompanied by use of UGA for tryptophan and, in ciliated protozoa, by use of UAA and UAG for glutamine. These are examples of "stop codon capture," which has been preceded by duplication of tRNA genes followed by nucleotide substitutions in their sequences, including mutational changes in their anticodons. Evolutionary changes in the code may have resulted from disappearance of codons and anticodons resulting from GC pressure and from their reappearance when the direction of the pressure was reversed. In this manner, codon UGA and anticodon UCA for tryptophan could have disappeared under GC pressure and reappeared in Mycoplasma under AT pressure. Stop codon UGA may have been the third of the three stop codons to appear, originating from mutations in UAA. Changes in the code are adaptive and nondeleterious. We propose that the number of anticodons has increased and that evolution continued until three existing forms of the universal code were produced: eukaryotic, eubacterial, and the code for halobacteria and methanococci. These three codes are distinguished from each other by their anticodon pattern. The eukaryotic code contains eight INN (ANN) anticodons that have replaced GNN anticodons as a result of AT pressure. Mitochondrial and chloroplast codes have evolved from the eubacterial code through genomic economization and AT pressure, leading to losses of GNN and CNN anticodons.(ABSTRACT TRUNCATED AT 250 WORDS)
通过比较各种生物体和细胞器中反密码子的使用情况，可以找到遗传密码进化的线索。由于定向突变压力（AT/GC压力），DNA的GC含量会发生变化，尤其是在细菌中。支原体中GC含量低伴随着UGA被用于编码色氨酸，而在纤毛原生动物中，UAA和UAG被用于编码谷氨酰胺。这些都是“终止密码子捕获”的例子，在此之前tRNA基因发生了复制，随后其序列发生了核苷酸替换，包括反密码子的突变变化。密码的进化变化可能是由于GC压力导致密码子和反密码子消失，以及当压力方向逆转时它们再次出现。通过这种方式，编码色氨酸的密码子UGA和反密码子UCA可能在GC压力下消失，并在支原体中在AT压力下再次出现。终止密码子UGA可能是出现的三个终止密码子中的第三个，起源于UAA的突变。密码的变化是适应性的且无害的。我们提出反密码子的数量增加了，并且进化持续进行，直到产生了通用密码的三种现有形式：真核生物、真细菌以及嗜盐菌和甲烷球菌的密码。这三种密码通过它们的反密码子模式相互区分。真核生物密码包含八个INN（ANN）反密码子，由于AT压力，它们取代了GNN反密码子。线粒体和叶绿体密码通过基因组节约化和AT压力从真细菌密码进化而来，导致GNN和CNN反密码子的丢失。（摘要截短为250字）
