Biosynthesis of anthraquinones by interspecies cloning of actinorhodin biosynthesis genes in streptomycetes: clarification of actinorhodin gene functions.

Streptomyces galilaeus ATCC 31133 and ATCC 31671, producers of the anthracyclines aclacinomycin A and 2-hydroxyaklavinone, respectively, formed an anthraquinone, aloesaponarin II, when they were transformed with DNA from Streptomyces coelicolor containing four genetic loci, actI, actIII, actIV, and actVII, encoding early reactions in the actinorhodin biosynthesis pathway. Subcloning experiments indicated that a 2.8-kilobase-pair XhoI fragment containing only the actI and actVII loci was necessary for aloesaponarin II biosynthesis by S. galilaeus ATCC 31133. Aloesaponarin II was synthesized via the condensation of 8 acetyl coenzyme A equivalents, followed by a decarboxylation reaction as demonstrated by [1,2-13C2]acetate feeding experiments. S. coelicolor B22 and B159, actVI blocked mutants, also formed aloesaponarin II as an apparent shunt product. Mutants of S. coelicolor blocked in several other steps in actinorhodin biosynthesis did not synthesize aloesaponarin II or other detectable anthraquinones. When S. galilaeus ATCC 31671 was transformed with the DNA carrying the actI, actIII, and actVII loci, the recombinant strain produced both aloesaponarin II and aklavinone, suggesting that the actinorhodin biosynthesis DNA encoded a function able to deoxygenate 2-hydroxyaklavinone to aklavinone. When S. galilaeus ATCC 31671 was transformed with a plasmid carrying only the intact actIII gene (pANT45), aklavinone was formed exclusively. These experiments indicate a function for the actIII gene, which is the reduction of the keto group at C-9 from the carboxy terminus of the assembled polyketide to the corresponding secondary alcohol. In the presence of the actIII gene, anthraquinones or anthracyclines formed as a result of dehydration and aromatization lack an oxygen function on the carbon on which the keto reductase operated. When S. galilaeus ATCC 31671 was transformed with the DNA carrying the actI, actVII, and actIV loci, the recombinant strain produced two novel anthraquinones, desoxyerythrolaccin, the 3-hydroxy analog of aloesaponarin II, and 1-O-methyldesoxyerythrolaccin. The results obtained in these experiments together with earlier data suggest a pathway for the biosynthesis of actinorhodin and related compounds by S. coelicolor.