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Abstract Neurospora crassa lysine auxotrophs 33933 and STL-7 which on nutritional grounds are known to be blocked at an early stage of lysine biosynthesis via the aminoadipic acid pathway, were found to accumulate homocitrate and malate plus citrate, respectively. Such accumulations predominate only when these mutants are grown in a medium containing minimal, but not excess, lysine. The acids were isolated by two-phase partition chromatography on silicic acid, and their structures were adduced from appropriate chemical and spectral data. Homocitrate was formed from acetate and α-ketoglutarate under suitable conditions by cell-free extracts of mutant 33933, but such homocitrate-condensing enzyme activity could not be shown in mutant STL-7 under the conditions of assay. Homocitrate-condensing enzyme activity of mutant 33933 extracts was subject to repression and feedback inhibition by lysine. The significance, if any, of malate and citrate accumulation to lysine metabolism in mutant STL-7 remains to be elucidated. This paper extends present knowledge, derived exclusively from studies in yeast, and shows that, in Neurospora, the initial step of lysine biosynthesis is via a homocitrate-condensing enzyme which is under effective end product control by lysine.
Hogg et al. (Mon,) studied this question.