Biochemical characterization of archaeal homocitrate synthase from Sulfolobus acidocaldarius

The hyperthermophilic archaeon, Sulfolobus, synthesizes lysine via the α-aminoadipate pathway; however, the gene encoding homocitrate synthase, the enzyme responsible for the first and committed step of the pathway, has not yet been identified. In the present study, we identified saci_1304 as the gene encoding a novel type of homocitrate synthase fused with a Regulation of Amino acid Metabolism (RAM) domain at the C terminus in Sulfolobus acidocaldarius. Enzymatic characterization revealed that Sulfolobus homocitrate synthase was inhibited by lysine; however, the mutant enzyme lacking the RAM domain was insensitive to inhibition by lysine. The present results indicated that the RAM domain is responsible for enzyme inhibition.