The tryptophane codon corresponding to UGA in Mycoplasma

密码子UGA是“通用遗传密码”中的翻译终止信号之一，在山羊支原体中被读作色氨酸（Yamao et al.，1985年）。这一结论基于以下观察。(1)克隆的核糖体蛋白基因簇的DNA序列。capriculum发现，许多UGA出现在阅读框架。在大多数情况下，UGA出现在相应的大肠杆菌核糖体蛋白中的色氨酸位置。(2)目前还没有发现UGA在M中被用作终止密码子。摩羯座(3)M. capricarpine含有两个<tRNA^（Trp）>，其中一个反密码子UCA对应密码子UGA，另一个反密码子CCA对应密码子UGG。支原体DNA的鸟嘌呤和胞嘧啶（G + C）含量极低（约25%）。该菌UGA密码子分配的改变可能是由于在进化过程中对支原体基因组DNA施加了以AT-对取代GC-对的偏向性突变压力所致。

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.