Neurospora mutant deficient in tryptophanyl-transfer ribonucleic acid synthetase activity

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Tryptophan biosynthesis is a crucial metabolic pathway regulated by feedback inhibition.
• Tryptophan auxotrophs, like trp-5 in Neurospora crassa, are essential tools for studying this regulation.
• The precise regulatory molecule (free tryptophan vs. charged tRNA) has been debated.

Purpose of the Study:

• To characterize the trp-5 mutant of Neurospora crassa.
• To elucidate the role of tryptophanyl-transfer RNA (tRNA) synthetase in regulating tryptophan biosynthesis.
• To determine whether free tryptophan or tryptophanyl-tRNA mediates feedback repression.

Main Methods:

• Enzyme activity assays for tryptophanyl-tRNA synthetase.
• Measurement of adenosine triphosphate-pyrophosphate exchange activity.
• Quantification of free tryptophan pools.
• Analysis of anthranilate synthetase and tryptophan synthetase levels.

Main Results:

• The trp-5 mutant exhibits less than 5% of wild-type tryptophanyl-tRNA synthetase activity.
• Adenosine triphosphate-pyrophosphate exchange activity is reduced by approximately 50% in the mutant.
• Derepressed tryptophan biosynthetic enzymes in the mutant correlate with normal or elevated free tryptophan levels.

Conclusions:

• The study identifies a deficiency in tryptophanyl-tRNA synthetase activity in the trp-5 mutant.
• Results indicate that tryptophanyl-tRNA, rather than free tryptophan, is the likely effector molecule for feedback repression of tryptophan biosynthetic enzymes.
• This finding clarifies a key regulatory mechanism in amino acid biosynthesis.