Tourancheau A B, Tsao N, Klobutcher L A, Pearlman R E, Adoutte A
Laboratoire de Biologie Cellulaire 4 (URA 1134), Université Paris-Sud, Orsay, France.
EMBO J. 1995 Jul 3;14(13):3262-7. doi: 10.1002/j.1460-2075.1995.tb07329.x.
In several species of ciliates, the universal stop codons UAA and UAG are translated into glutamine, while in the euplotids, the glutamine codon usage is normal, but UGA appears to be translated as cysteine. Because the emerging position of this monophyletic group in the eukaryotic lineage is relatively late, this deviant genetic code represents a derived state of the universal code. The question is therefore raised as to how these changes arose within the evolutionary pathways of the phylum. Here, we have investigated the presence of stop codons in alpha tubulin and/or phosphoglycerate kinase gene coding sequences from diverse species of ciliates scattered over the phylogenetic tree constructed from 28S rRNA sequences. In our data set, when deviations occur they correspond to in frame UAA and UAG coding for glutamine. By combining these new data with those previously reported, we show that (i) utilization of UAA and UAG codons occurs to different extents between, but also within, the different classes of ciliates and (ii) the resulting phylogenetic pattern of deviations from the universal code cannot be accounted for by a scenario involving a single transition to the unusual code. Thus, contrary to expectations, deviations from the universal genetic code have arisen independently several times within the phylum.
在几种纤毛虫中，通用终止密码子UAA和UAG被翻译成谷氨酰胺，而在游仆虫中，谷氨酰胺密码子的使用是正常的，但UGA似乎被翻译成半胱氨酸。由于这个单系类群在真核生物谱系中的出现位置相对较晚，这种异常的遗传密码代表了通用密码的一种衍生状态。因此，问题就出现了，这些变化是如何在该门的进化途径中产生的。在这里，我们研究了分布在由28S rRNA序列构建的系统发育树上的不同纤毛虫物种的α微管蛋白和/或磷酸甘油酸激酶基因编码序列中终止密码子的存在情况。在我们的数据集中，当出现偏差时，它们对应于编码谷氨酰胺的框内UAA和UAG。通过将这些新数据与先前报道的数据相结合，我们表明：（i）UAA和UAG密码子的使用在不同类别的纤毛虫之间以及同一类别内部都有不同程度的发生；（ii）由此产生的与通用密码偏差的系统发育模式不能用涉及单次转变为异常密码的情况来解释。因此，与预期相反，在该门内，与通用遗传密码的偏差已经独立出现了好几次。
