第一个被确定的密码子是ITT,对应Phe 1961年, Marshall nirenberg用PP合成了 poly(U),将它放入20根试管中,每根试管中 加入了大肠杆菌细胞质抽提物和一种有同位 素标记的氨基酸。反应一段时间后用TCA (三氯乙酸)将蛋白质沉淀下来,测定同位 素的量,结果发现在加了同位素标记Phe的 试管的沉淀物中同位素含量特别高。 很快,密码子CC(对应Pro,AAA对应Iy,但 poly(G没有得到结果(高级结构的缘故)
• 第一个被确定的密码子是TTT，对应Phe. 1961年，Marshall Nirenberg用PP合成了 poly(U), 将它放入20根试管中，每根试管中 加入了大肠杆菌细胞质抽提物和一种有同位 素标记的氨基酸。反应一段时间后用TCA （三氯乙酸）将蛋白质沉淀下来，测定同位 素的量，结果发现在加了同位素标记Phe的 试管的沉淀物中同位素含量特别高。 • 很快，密码子CCC对应Pro, AAA对应Lys, 但 poly(G)没有得到结果（高级结构的缘故）
table 27-1 Incorporation of Amino Acids into Polypeptides in Response to random polymers of rna* Expected freq 用统计 Observed Tentative assignment of incorporation frequency of for nucleotide based on 法尝试 incorporation composition of assignment Amino acid (Lys= 100) corresponding codon (Lys= 100) 确定含 Asparagine 24 A2C 20 Glutamine 24 AC 多种核 Histidine 6 4 苷酸的hame 100 AA 100 AC, CCC 4.8 密码子 Threonine 26 A2C, AC? 24 的碱基 Presented here is a summary of data from one of the early experiments designed to elucidate he genetic code. A synthetic RNA containing only A and C residues in a 5: 1 ratio directed 组份 polypeptide synthesis. Both the identity and the quantity of amino acids incorporated were determined Based on the relative abundance of A and c residues in the synthetic RNA, and assigning the codon AAa (the most likely codon)a frequency of 100, there should be three different codons of composition A2 C, each at a relative frequency of 20; three codons of composition AC2, each at a relative frequency of 4.0; and codon CCC at a relative frequency of 0.8. The CCC assignment was based on information derived from prior studies with poly(C). Where two tentative codon assignments are made, both are proposed to code for the same amino acid TNote that these designations of nucleotide composition contain no information on nucleotide sequence(except, of course, AAA and CCC
用统计 法尝试 确定含 多种核 苷酸的 密码子 的碱基 组份
1964年 Nirenberg等发现将核苷酸三联体与对应 的AA-tRNA放在一起,AA-RNA就会结合到 核糖体上去。 able 27-2 Trinucleotides Induce Specific Binding of Aminoacyl-tRNAs to ribosomes 14C-Labeled aminoacyl-tRNa bound to ribosome* Trinucleotide Phe-tRNAPhe Lys-tRNA-ys Pro-tRNAPTo UUU 400 0 0 AAA 7.7 0 CCC 3.1 Source: Modified from Nirenberg, M.& Leder, P.(1964)RNA code words and protein synthesis Science 145, 1399 Each number represents the factor by which the amount of bound ic increased when the indicated trinucleotide was present, relative to a control in which no trinucleotide was added
1964年Nirenberg等发现将核苷酸三联体与对应 的AA-tRNA放在一起，AA-tRNA就会结合到 核糖体上去
Radioactive amino acid Trinucleotide 核糖体结合 Ribosome Filter 实验 The final Let solution val Phe
核糖体结合 实验: The final solution
几乎同时, Gobind table 27-3 Polypeptides Produced in Response to Khorana成功地合成了 Synthetic RNA Polymers with Repeating Sequences of Three and Four Bases 非随机重复的多聚核苷 Polynucleotide Polypeptide products Trinucleotide repeats 酸。当这些多聚物得到|00 UUG)n Cys)o, (Val)n 的结果与 Nirenberg的uwm 结果进行比较时,得到mmH (GAU) (Asp)o( Met),(chain terminator) 了确切的密码子信息。 Tetranucleotide repeats (UAUC) (Tyr-Leu-Ser-lle)o 如(AC得到的结果是 (Leu-Leu-Thr-Tyr)n Di-and tripeptides" H和T等量参入,而密 CAUAG) Di- and tripeptides* with these polynucleotides, the patterns of amino acid 码的分布是等量的CAC 和ACA,因此CAC对应 reading frames includes only termination codons and thus only two homopolypeptides are observed (generated from the remaining two read ing frames). In some of the repeating tetranucleotide sequences, every fourth codon is a termination H,ACA对应T codon in every reading frame, so only short peptides are produced. This is illustrated in Figure 27-6 for (GUAA)n
几乎同时，Gobind Khorana成功地合成了 非随机重复的多聚核苷 酸。当这些多聚物得到 的结果与 Nirenberg的 结果进行比较时，得到 了确切的密码子信息。 如 (AC)n得到的结果是 H和T等量参入，而密 码的分布是等量的CAC 和ACA,因此CAC对应 H, ACA对应T