The anthranilate synthetase enzyme complex and the trifunctional trpC gene of Aspergillus.

Abstract

In all fungi, anthranilate synthetase (AS) depends on two unlinked genes, one coding for a protein which has ammonia-dependent AS activity in vitro, the other providing glutamine-amido transferase (GAT) activity. In Aspergillus nidulans these proteins are part of a 10 S enzyme complex; the former is the trpA gene product and can be isolated as 4.5 S subunits, while the latter is one of three enzyme activities of the 7.5 S subunits coded for by the trpC gene or gene region. To investigate whether trpC represents a multi-functional gene or an operon-like gene cluster various trpC mutants were mapped by meiotic recombination, checked for complementation in diploids, assayed for residual enzyme activities and the size of their active proteins was estimated by sucrose gradient centrifugation. Three of the five types found each lacked one enzyme activity, produced 10 S complexes and complemented each other; mutants of the same type mapped in the same gene segment and presumably were missense mutations. The most extreme type lacked all three activities but several cases showed partial complementation, indicative of a one-gene trifunctional-polypeptide situation. However, the most interesting type retained GAT-function alone and produced AS-activity in 6-7 S complexes. These are proposed to contain trpC polypeptide fragments which fold normally in the GAT-domain and by combining with trpA+polypeptides form complexes of reduced but almost constant size due to protease activities, as demonstrated for the homologous proteins in Neurospora by Hulett and DeMoss (J. Biol. Chem. 250: 6648, 1975).