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Neurospora Mutant Deficient in Tryptophanyl-Transfer Ribonucleic Acid Synthetase Activity

^a Department of Biochemistry, University of Kansas Medical Center, Kansas City, Kansas 66103
^b Department of Microbiology, University of Kansas Medical Center, Kansas City, Kansas 66103
Department of Biology, University of Missouri, Kansas City, Missouri 64110

ABSTRACT

A tryptophan auxotroph of Neurospora crassa, trp-5, has been characterized as a mutant with a deficient tryptophanyl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.2) activity. When assayed by tryptophanyl-tRNA formation, extracts of the mutant have less than 5% of the wild-type specific activity. The adenosine triphosphate-pyrophosphate exchange activity is at about half the normal level. In the mutant derepressed levels of anthranilate synthetase and tryptophan synthetase were associated with free tryptophan pools equal to or higher than those found in the wild type. We conclude that a product of the normal tryptophanyl-tRNA synthetase, probably tryptophanyl-tRNA, rather than free tryptophan, participates in the repression of the tryptophan biosynthetic enzymes.

¹ Senior Foreign Scientist Fellow of the National Science Foundation. On leave from the Department of Botany, University of Dacca, Pakistan.