Genetics and Biochemistry of the Cysteine-Tyrosine Relationship in Neurospora crassa

Citation

Shen, San-Chiun (1951) Genetics and Biochemistry of the Cysteine-Tyrosine Relationship in Neurospora crassa. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BE55-1F07. https://resolver.caltech.edu/CaltechETD:etd-06282004-093704

Abstract

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Mutant 84605 which was obtained from wild type Neurospora crassa following X-ray treatment differs from wild type by a single gene located 4.8 units from the centromere of the second chromosome.

At 25 [degrees], the mutant requires both cysteine and tyrosine for normal growth. At 35[degrees] only cysteine is required. The block in cysteine synthesis is the step sulfite [...] thiosulfate.

High tyrosinase activity was found in the mutant grown at 25[degrees], but not when grown at 35[degrees]. Under the same conditions, wild type shows little or no tyrosinase activity.

The addition of cysteine to the medium causes an inhibition of the growth of wild type, and at the same time a marked increase in the tyrosinase activity occurs. The inhibition can be overcome by adding tyrosine to the medium, or by culturing at 35[degrees].

It is suggested that the tyrosine requirement is caused by the high tyrosinase activity and that the latter, in turn, is caused by the defect in sulfur metabolism.

Two natural inhibitors of tyrosinase have been found in Neurospora.

A powerful inhibitor of the growth of wild type accumulates in cultures of the mutant.

Experiments designed to test whether sulfide can serve as a sulfur source for Neurospora indicate that sulfide is utilized slightly, if at all.

Experiments with a double mutant have indicated that the production of cysteine from methionine by Neurospora does not involve simple reversal of the step homocysteine [...] methionine.

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