Mutant ED-1369 from demethyltetracycline-producing Streptomyces aureofaciens strains exhibits rust-colored pigmentation, low antibacterial activity (<1 mpg/ml demethylchlortetracycline), capability to convert known tetracycline biosynthetic intermediates, and cosynthetic response with other noncoincident point-blocked mutants, being regarded as a "universal acceptor" blocked at an early stage of tetracycline biosynthesis. To investigate tetracycline-related metabolites in ED-1369, its mycelial pigment was studied, leading to the isolation of 9,10-dihydro-4,5-dihydroxy-3-malonamoyl-9,10-dioxo-2-anthraceneacetic acid (protetrone) via extraction into acidic tetrahydrofuran, fractional precipitation with hexane, conversion to sodium salt and back to free acid, and recrystallization from acidified dimethyl sulfoxide-methylene chloride. Protetrone is an orange crystalline solid (mp 186-190° dec, C19H13NO8) characterized by UV, IR, and NMR spectra. Chemical reactions confirmed its structure: zinc dust distillation yielded anthracene, dehydration in HBr-acetic acid gave a naphthaquinone-like compound, refluxing in 58% H2SO4-phenol produced pretetramid (30% yield), decarboxylation in DMSO formed an anthraquinone, and thermal degradation yielded another anthraquinone. Protetrone showed no biological activity as a tetracycline precursor, indicating it is not an intermediate but a shunt product arising from oxidation of the corresponding anthrone due to a block in the final cyclization to pretetramid in ED-1369. As an incompletely cyclized polyketide, protetrone suggests that earlier intermediates or structurally significant shunt products may accumulate in blocked-mutant fermentations.