Murray J A, Cesareni G, Argos P
European Molecular Biology Laboratory, Heidelberg Federal Republic of Germany.
J Mol Biol. 1988 Apr 5;200(3):601-7. doi: 10.1016/0022-2836(88)90546-3.
Four closely related species of yeast possess multicopy nuclear plasmids whose shared molecular architecture demonstrates a common ancestor, despite their lack of discernible DNA sequence homology. Each plasmid encodes three proteins which have equivalent essential functions in plasmid maintenance. These three groups of proteins show markedly different degrees of conservation, so that although we have successfully aligned sequences for two groups, members of the third group have diverged to such an extent that they cannot be aligned. All the proteins are sufficiently different that they function only in conjunction with their encoding plasmid. These proteins have therefore conserved their functional interactions with the relevant DNA sequences of their particular plasmids, despite lack of amino acid sequence conservation. The maintenance of function in the face of DNA sequence divergence is analogous to the coevolution of ribosomal DNA promoters and RNA polymerase I, and suggests that molecular drive may be an important force in the evolution of these plasmids. This view is reinforced by the inconsistent phylogenetic relationships determined from the two alignment sets, and by the contradiction that the two plasmids known to be the closest related taxonomically and by their host interchangeability are suggested to be the most distant by their sequences.
四种密切相关的酵母物种拥有多拷贝核质粒，尽管它们缺乏可识别的DNA序列同源性，但其共享的分子结构表明它们有一个共同的祖先。每个质粒编码三种蛋白质，这些蛋白质在质粒维持中具有同等重要的功能。这三组蛋白质显示出明显不同程度的保守性，因此尽管我们成功地比对了两组序列，但第三组的成员已经分化到无法比对的程度。所有这些蛋白质都差异很大，以至于它们仅与其编码质粒协同发挥作用。因此，尽管缺乏氨基酸序列保守性，但这些蛋白质与其特定质粒的相关DNA序列保持了功能上的相互作用。面对DNA序列差异时功能的维持类似于核糖体DNA启动子和RNA聚合酶I的共同进化，并表明分子驱动力可能是这些质粒进化中的一个重要力量。从两组比对中确定的不一致的系统发育关系，以及分类学上已知最密切相关且宿主可互换的两个质粒却被其序列表明是最不相关的这一矛盾，都强化了这一观点。
