Bowen T L, Union J, Tumbula D L, Whitman W B
Department of Microbiology, University of Georgia, Athens 30602-2605, USA.
Gene. 1997 Mar 25;188(1):77-84. doi: 10.1016/s0378-1119(96)00779-2.
The gene for acetohydroxyacid synthase (AHAS) was cloned from the archaeon Methanococcus aeolicus. Contrary to biochemical studies [Xing, R. and Whitman, W.B. (1994) J. Bacteriol. 176, 1207-1213] the enzyme was encoded by two open reading frames (ORFs). Based on sequence homology, these ORFs were designated ilvB and ilvN for the large and small subunits of AHAS, respectively. A putative methanogen promoter preceded ilvB-ilvN, and a potential internal promoter was found upstream of ilvN. ilvB encoded a 65-kDa protein, which agreed well with the measured value for the purified enzyme. ilvN encoded a 19-kDa protein, which fell within the range of M(r) of small subunits from other sources. Phylogenetic analysis of the deduced amino acid sequence of ilvB showed a close relationship between the AHAS of Bacteria and Archaea, to the exclusion of other enzymes in this family, including pyruvate oxidase, glyoxylate carboligase, pyruvate decarboxylase, and the acetolactate synthase found in fermentative Bacteria. Thus, this family of enzymes probably arose prior to the divergence of the Bacteria and Archaea. Moreover, the higher plant AHAS and the red algal AHAS were related to the AHAS II of Escherichia coli and the cyanobacterial AHAS, respectively. For this reason, these genes appear to have been acquired by the Eucarya during the endosymbiosis that gave rise to the mitochondrion and chloroplast, respectively. One of the ORFs in the Methanococcus jannaschii genome possesses high similarity to the M. aeolicus ilvB, indicating that it is an authentic AHAS.
从嗜热栖热甲烷球菌中克隆出乙酰羟酸合酶（AHAS）基因。与生化研究结果[邢，R.和惠特曼，W.B.（1994年）《细菌学杂志》176，1207 - 1213]相反，该酶由两个开放阅读框（ORF）编码。基于序列同源性，这些ORF分别被命名为ilvB和ilvN，代表AHAS的大亚基和小亚基。ilvB - ilvN之前有一个假定的产甲烷菌启动子，并且在ilvN上游发现了一个潜在的内部启动子。ilvB编码一个65 kDa的蛋白质，这与纯化酶的测量值非常吻合。ilvN编码一个19 kDa的蛋白质，其分子量在其他来源的小亚基分子量范围内。对ilvB推导的氨基酸序列进行系统发育分析表明，细菌和古菌的AHAS之间关系密切，该家族中的其他酶被排除在外，包括丙酮酸氧化酶、乙醛酸羧化酶、丙酮酸脱羧酶以及在发酵细菌中发现的乙酰乳酸合酶。因此，这个酶家族可能在细菌和古菌分化之前就已经出现。此外，高等植物的AHAS和红藻的AHAS分别与大肠杆菌的AHAS II和蓝细菌的AHAS相关。因此，这些基因似乎是真核生物在分别导致线粒体和叶绿体的内共生过程中获得的。詹氏甲烷球菌基因组中的一个ORF与嗜热栖热甲烷球菌的ilvB具有高度相似性，表明它是一个真正的AHAS。
