5-Hydroxyaloesaponarin II, a minor blue pigment in an actinorhodin-negative mutant of Streptomyces coelicolor A3(2)

Blue pigmentation in Streptomyces coelicolor A3(2) is attributed to synthesis of the polyketide actinorhodin and its lactone derivative γ-actinorhodin, Therefore, actinorhodin-negative mutants show pigmentation other than blue. When the B22 mutant of S.coelicolor A3(2) [defective in the actVI-ORF1 gene coding for a putative keto( = oxo)reductase] was examined for its secondary metabolite content, the presence of aloesaponarin II (3,8-dihydroxy-1-methyl-9,10-anthraquinone) as the major pigment was confirmed. However, a substantial fraction of a red/blue (acid/alkaline) pigment was detected after separation on HPLC. MS and NMR analysis revealed its structure as 3,5,8-trihydroxy-1-methyl-9,10-anthraquinone. To our knowledge, this anthraquinone has not previously been reported in biological material. A possible route for biosynthesis of this compound is discussed.