Aspergillus nidulans exhibited high γ-glutamyl transpeptidase (γGT) activity in both carbon-starved and carbon-limited cultures. Glucose repressed, but casein peptone increased γGT production. Null mutation of creA did not influence γGT formation, but the functional meaB was necessary for the γGT induction. Deletion of the AN10444 gene (ggtA) completely eliminated the γGT activity, and the mRNA levels of ggtA showed strong correlation with the observed γGT activities. While ggtA does not contain a canonical signal sequence, the γGT activity was detectable both in the fermentation broth and in the hyphae. Deletion of the ggtA gene did not prevent the depletion of glutathione observed in carbon-starved and carbon-limited cultures. Addition of casein peptone to carbon-starved cultures lowered the formation of reactive species (RS). Deletion of ggtA could hinder this decrease and resulted in elevated RS formation. This effect of γGT on redox homeostasis may explain the reduced cleistothecia formation of ΔggtA strains in surface cultures.

Relative transcriptions of Aspergillus nidulans dug1-3 (orthologes of Saccharomyces cerevisiae DUG -deficient in utilization of glutathione -pathway genes) and ggtA encoding g-glutamyl transpeptidase were studied under conditions inducing glutathione degradation. GgtA was induced in all cases when glutathione levels decreased, but addition of yeast extract, which moderated glutathione degradation, enhanced its induction. Although dug2 showed constitutive transcription, dug1 and dug3 were induced by carbon and nitrogen starvation and yeast extract did not caused significant changes in their relative transcription. The in silico reconstructed DUG pathway of A. nidulans is a promising candidate for cytosolic GSH degradation induced by carbon/nitrogen stress.

Aspergillus nidulans γ-glutamyl transpeptidase (AnγGT, EC 2.3.2.2) was partially purified from the fermentation broth of carbon stressed cultures. Its temperature and pH optimum was 45 ºC and pH 8.0, respectively. AnγGT had little hydrolase activity. It utilized Gln, glutathione and less efficiently oxidized glutathione as γ-glutamyl donors (beside of γ-glutamyl-p-nitroanilide) and amino-acids and peptides (including Glu, Cys, Met, Gly-Gly and Cys-Gly) but not hydroxylamine as γ-glutamyl acceptors. We propose that the function of this enzyme is not to degrade, but to produce, γ-glutamyl compounds which may be related to the utilization of extracellular peptides and aminoacids in carbon stressed cultures.