Cloning and phylogenetic analysis of the genes encoding acetohydroxyacid synthase from the archaeon Methanococcus aeolicus

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.