Clarke C F, Williams W, Teruya J H
Department of Medicine, UCLA School of Medicine 90024.
J Biol Chem. 1991 Sep 5;266(25):16636-44.
Ubiquinone (or coenzyme Q) is a lipid component of the respiratory chain in the inner mitochondrial membrane, in which it functions in electron transport. Recent reports show that ubiquinone and ubiquinone biosynthetic enzymes are present in both mitochondrial and nonmitochondrial membranes of cells (Kalen, A., Appelkvist, E.-L., Chojnacki, T., and Dallner, G. (1990) J. Biol. Chem. 265, 1158-1164) although the functions that ubiquinone may play outside of the mitochondrion are not understood. To study coenzyme Q synthesis and function we cloned the 3,4-dihydroxy-5-hexaprenylbenzoate (DHHB) methyltransferase gene by functional complementation of a yeast coenzyme Q mutant strain, defective in the COQ3 gene (Tzagoloff, A., and Dieckmann, C. L. (1990) Microbiol. Rev. 54, 211-225). This gene restores both coenzyme Q synthesis in the mutant strain and the ability to grow on media containing glycerol, a nonfermentable substrate. A one-step in situ gene replacement with the cloned DHHB methyltransferase DNA directs integration to the yeast COQ3 locus on chromosome XV of Saccharomyces cerevisiae, establishing that the COQ3 locus encodes the DHHB methyltransferase structural gene. The predicted amino acid sequence of the yeast DHHB methyltransferase contains a methyltransferase consensus sequence and shows a 40% identity with an open reading frame of Escherichia coli, the gyrA5' hypothetical protein. This open reading frame is adjacent to the gyrA gene and close to the mapped location of the ubiG gene at 48 min on the E. coli chromosome. These results suggest that the E. coli gyrA5' open reading frame encodes a methyltransferase and may correspond to the ubiG gene, which is required for ubiquinone biosynthesis.
泛醌（或辅酶Q）是线粒体内膜呼吸链的一种脂质成分，在其中参与电子传递。最近的报道表明，泛醌和泛醌生物合成酶存在于细胞的线粒体膜和非线粒体膜中（卡伦，A.，阿佩尔奎斯特，E.-L.，乔伊纳茨基，T.，和达尔纳，G.（1990年）《生物化学杂志》265卷，第1158 - 1164页），尽管泛醌在线粒体外可能发挥的功能尚不清楚。为了研究辅酶Q的合成和功能，我们通过对酵母辅酶Q突变株（该突变株在COQ3基因上有缺陷，扎戈洛夫，A.，和迪克曼，C.L.（1990年）《微生物学评论》54卷，第211 - 225页）进行功能互补，克隆了3,4 - 二羟基 - 5 - 六异戊烯基苯甲酸（DHHB）甲基转移酶基因。该基因恢复了突变株中的辅酶Q合成以及在含有甘油（一种非发酵底物）的培养基上生长的能力。用克隆的DHHB甲基转移酶DNA进行一步原位基因置换，可将其整合到酿酒酵母第十五号染色体上的酵母COQ3基因座，这表明COQ3基因座编码DHHB甲基转移酶结构基因。酵母DHHB甲基转移酶的预测氨基酸序列包含一个甲基转移酶共有序列，并且与大肠杆菌的一个开放阅读框（gyrA5'假设蛋白）有40%的同一性。这个开放阅读框与gyrA基因相邻，且接近大肠杆菌染色体上48分钟处ubiG基因的定位位置。这些结果表明，大肠杆菌gyrA5'开放阅读框编码一种甲基转移酶，可能对应于泛醌生物合成所需的ubiG基因。
