Pesole G, Gissi C, Lanave C, Saccone C
Dipartimento di Biochimica e Biologia Molecolare, Universitá di Bari, Italy.
Mol Biol Evol. 1995 Mar;12(2):189-97. doi: 10.1093/oxfordjournals.molbev.a040197.
The evolution of the prokaryotic glutamine synthase (GS) genes, namely the GSI and GSII isoforms, has been investigated using the second codon positions, which have previously proven to behave as a good molecular clock. Our data confirm the early divergence between prokaryotic and eukaryotic GSII before the splitting between plants and animals. The phylogenetic tree of the GSI isoforms shows Archaebacteria to be more closely related to Eubacteria than to Eukaryotes. This finding is confirmed by the phylogenetic analysis carried out on both large and small subunits of rRNA. However, differently from the rRNA analyses, Crenarchaeota and Euryarchaeota Archaebacteria, as well as high- and low-GC gram-positive bacteria, appear to be polyphyletic. We provide evidence that the observed polyphyly of Archaebacteria might be only apparent, resulting from a gene duplication event preceding the split between Archaebacteria and Eubacteria and followed by the retention of only one isoform in the extant lineages. Both gram-negative bacteria and high-GC gram-positive bacteria, which appear closely related, have GS activity regulated by an adenylylation/deadenylylation mechanism. A lateral gene transfer from Archaebacteria to low-GC eubacteria is invoked to explain the observed polyphyly of gram-positive bacteria.
利用第二个密码子位置对原核生物谷氨酰胺合成酶（GS）基因，即GSI和GSII亚型的进化进行了研究，此前已证明第二个密码子位置可作为一个良好的分子钟。我们的数据证实了在植物和动物分化之前，原核生物和真核生物GSII之间的早期分化。GSI亚型的系统发育树显示古细菌与真细菌的关系比与真核生物的关系更密切。对rRNA的大亚基和小亚基进行的系统发育分析证实了这一发现。然而，与rRNA分析不同的是，泉古菌门和广古菌门的古细菌，以及高GC和低GC革兰氏阳性菌似乎是多系的。我们提供的证据表明，观察到的古细菌多系现象可能只是表面现象，这是由古细菌和真细菌分化之前的一次基因复制事件导致的，随后在现存谱系中仅保留了一种亚型。革兰氏阴性菌和高GC革兰氏阳性菌看起来关系密切，它们的GS活性都由腺苷酸化/去腺苷酸化机制调节。有人提出从古细菌到低GC真细菌的横向基因转移来解释观察到的革兰氏阳性菌多系现象。
