A cloned tryptophan-synthesis gene from the ascomycete Cochliobolus heterostrophus functions in Escherichia coli, yeast and Aspergillus nidulans.

A gene (TRP1) in the tryptophan biosynthetic pathway of the fungal plant pathogen Cochliobolus heterostrophus was isolated by complementation of an Escherichia coli trpF mutant which lacked phosphoribosylanthranilate isomerase (PRAI) activity. The cloned gene also complemented an E. coli trpC mutant lacking indoleglycerolphosphate synthase (IGPS) activity, a yeast trp1 mutant missing PRAI activity and an Aspergillus nidulans trpC mutant. It functioned in E. coli and A. nidulans without apparent rearrangement but in yeast only after the 5' end of the gene was deleted. The gene was subcloned on a 4.65-kb DNA fragment and the PRAI domain was localized to a 2.9-kb region. It showed homology to the A. nidulans trpC and Neurospora crassa trp-1 genes. There was one predominant transcript of C. heterostrophus TRP1, the same size (2.6-kb) as one of the two functional transcripts produced by A. nidulans trpC. The constitutive activity of the C. heterostrophus TRP1 gene was high whereas that of the A. nidulans trpC gene was low.