Pathways of Cysteine Synthesis in Aspergillus nidulans

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Abstract
A combination of metabolic and mutant studies was used to elucidate the pathways. Fresh mycelium synthesizes cysteine only slowly and variably, but acetone powders of mycelium prepared at low temperatures were satisfactory and were used throughout the work. Cysteine was synthesized from sulfate and sulfite on the addition of pyridoxal, and the synthesis was further enhanced by glutamate and pyruvate. Cysteine synthesis from thiosulfate was increased by the addition of serine. Cysteine sulfinic acid was produced when sulfite was the inorganic source. Growth studies with a number of mutants indicated that the metabolic block is between sulfate and sulfite. Two reaction sequences are proposed; sulfate - sulfite - cysteine sulfinic acid -cysteine; and thiosulfate - cysteine-S-sulfonic acid - cysteine.