The tryptophane codon corresponding to UGA in Mycoplasma

密码子UGA是“通用遗传密码”中的翻译停止信号之一，在支原体中被解读为色氨酸（Yamao et al., 1985）。这一发现基于以下观察。(1)克隆的山羊支原体核糖体蛋白基因簇DNA序列显示，在阅读框中出现了许多UGA。在大多数情况下，UGAs出现在大肠杆菌核糖体蛋白色氨酸的位置。(2)到目前为止，没有发现UGA在M. capricolum中用作终止密码子。(3) M. capricolum含有两个<tRNA^(Trp)>，其中一个具有与密码子UGA对应的反密码子UCA，另一个具有与通用色氨酸密码子UGG对应的CCA。一些观察表明，UGA是色氨酸的主要密码子，UGG在山羊分枝杆菌中很少使用。支原体DNA的鸟嘌呤和胞嘧啶（G + C）含量极低（约25%）。该细菌UGA密码子分配的变化可能是由于进化过程中施加在支原体基因组DNA上的at对取代gc对的偏突变压力引起的。

The codon UGA, one of the translational stop signals in the "universal genetic code", is read as tryptophane in Mycoplasma capricolum (Yamao et al., 1985). This finding is based on the following observations. (1) The DNA sequence of the cloned ribosomal protein gene cluster of M. capricolum revealed that many UGA appeared in the reading frames. In most cases, the UGAs appeared at the position of tryptophane in the corresponding Escherichia coli ribosomal proteins. (2) No UGA is so far found to be used as a stop codon in M. capricolum. (3) M. capricolum contains two <tRNA^(Trp)> : one of which has an anticodon UCA corresponding to the codon UGA, and the other has CCA for universal tryptophane codon UGG.Several observations indicated that UGA is the predominant codon for tryptophane and UGG is rarely used in M. capricolum.The guanine and cytosine (G + C) content of Mycoplasma DNA is extremely low (about 25%). The change of the UGA codon assignment in this bacterium might be brought about by the biased mutation pressure replaceing GC-pairs by AT-pairs that had been exerted on the mycoplasma genomic DNA during evolution.