Involved in the biosynthesis of aklavinone which is an important precursor common to the formation of the clinically significant anthracyclines such as carminomycin, daunorubicin (daunomycin), rhodomycin, aclacinomycin T (aklavin) and aclacinomycin A (aclarubicin). These compounds are aromatic polyketide antibiotics that exhibit high cytotoxicity and are widely applied in the chemotherapy of a variety of cancers. Catalyzes the cyclization of aklanonic acid methyl ester to yield aklaviketone presumably via an intramolecular aldol condensation mechanism, although water is not eliminated. methyl aklanonate = aklaviketone Optimum pH is 7. The enzyme is active between pH 5.0 and 9.0. Antibiotic biosynthesis; daunorubicin biosynthesis. Antibiotic biosynthesis; carminomycin biosynthesis. Antibiotic biosynthesis; rhodomycin biosynthesis. Antibiotic biosynthesis; aclacinomycin biosynthesis. Homotetramer. In contrast to the analogous intramolecular aldol cyclization catalyzed by TcmI, the conversion catalyzed by DnrD occurs after anthraquinone formation and requires activation of a carboxylic acid group by esterification of aklanonic acid, the aklanonic acid methyl ester precursor. Belongs to the polyketide cyclase DnrD family.