In Thermus thermophilus homocitrate synthase (HCS) catalyzes the initial reaction of lysine biosynthesis through alpha-aminoadipic acid, synthesis of homocitrate from 2-oxoglutarate and acetyl-CoA. HCS is strongly inhibited by lysine, indicating that the biosynthesis is regulated by the endproduct at the initial reaction in the pathway. HCS also catalyzes the reaction using oxaloacetate in place of 2-oxoglutarate as a substrate, similar to citrate synthase in the tricarboxylic acid cycle. Several other properties of Thermus HCS and an evolutionary relationship of the biosynthetic pathway in the bacterium to other metabolic pathways are also described.