Organization of Enzymes in the Common Aromatic Synthetic Pathway: Evidence for Aggregation in Fungi

Abstract

Centrifugation in sucrose density gradients of partially purified extracts from six species of fungi, i.e., Rhizopus stolonifer, Phycomyces nitens, Absidia glauca ( Phycomycetes ), Aspergillus nidulans ( Ascomycetes ), Coprinus lagopus , and Ustilago maydis ( Basidiomycetes ), indicate that the five enzymes catalyzing steps two to six in the prechorismic acid part of the polyaromatic synthetic pathway sediment together. The sedimentation coefficients for these enzymes are very similar in the six species and are comparable to those previously observed for the multienzyme complexes ( arom aggregates) of Neurospora crassa and Saccharomyces cerevisiae . These results are interpreted as indicating the presence in each of these fungi of arom aggregates, presumably encoded by arom gene clusters similar to those in N. crassa and S. cerevisiae . Evidence has also been obtained for the presence in two species ( A. nidulans and U. maydis ) and the absence in the other four species of a second dehydroquinase isozyme which is distinguishable from the synthetic activity on the basis of both thermostability tests and S values. This second dehydroquinase, which is apparently involved in the catabolism of quinic acid via a pathway similar to that in N. crassa , is inducible in A. nidulans (as it is in N. crassa ), but constitutive in U. maydis . These comparative findings are discussed in relation to the organization, evolution, and possible functional relationships of synthetic and catabolic aromatic pathways in fungi.