Tryptophan biosynthesis in Coprinus lagopus: a genetic analysis of mutants

Thirty-one tryptophan-requiring mutants of Coprinus lagopus have been assigned by genetic and complementation analyses to four loci designated trp-I, trp-2, trp-3 and trp-4. The trp-1 and trp-3 loci were located in group III and trp-2 in group G of the linkage map. The trp-4 locus showed no linkage to the other trp loci or to markers in three additional linkage groups tested. From auxanographic tests and a study of accumulated biosynthetic intermediates, the enzymes controlled by each locus have been provisionally assigned. The trp-2 and trp-3 loci both appear necessary for anthranilate synthetase activity since mutants accumulated no intermediates. Only the trp-3 mutant could utilize anthranilic acid, therefore the trp-2 locus must also be involved in a subsequent step in the pathway. The trp-4 mutants utilized indole and accumulated anthranilic acid, and hence this locus is involved in the conversion of anthranilic acid to indoleglycerol phosphate. The trp-1 mutants utilized only tryptophan and accumulated indoleglycerol phosphophate and anthranilic acid. They are therefore blocked in the final steps of the pathway catalysed by tryptophan synthetase.