ENGINEERED BIOSYNTHESIS OF NOVEL POLYKETIDES - STEREOCHEMICAL COURSE OF 2 REACTIONS CATALYZED BY A POLYKETIDE SYNTHASE

A genetically engineered strain expressing the essential components of the tetracenomycin polyketide synthase (tcm PKS) along with the actinorhodin ketoreductase (act KR) was found to produce two new (diastereomeric) aromatic polyketides, designated RM20b and RM20c, in addition to RM20, whose structure was reported earlier [McDaniel, R., Ebert-Khosla, S., Hopwood, D. A., and Khosla, C. (1993) Science 262, 1546-1550]. Spectroscopic and in vivo isotopic labeling analysis of RM20b and RM20c revealed that their polyketide backbones were identical to that of RM20 with respect to chain length, regiospecificity of ketoreduction, and regiospecificity of the first intramolecular aldol condensation. This is consistent with earlier predictions that the essential components of the PKS-a bifunctional ketosynthase/acyltransferase, a chain length determining factor, and an acyl carrier protein-are responsible for controlling each of these features of the polyketide backbone [McDaniel, R., Ebert-Khosla, S., Hopwood, D. A., and Khosla, C. (1993) Science 262, 1546-1550; McDaniel, R., Ebert-Khosla, S., Hopwood, D. A., and Khosla, C. (1993) J. Am. Chem. Sec. 115, 11671-11675; Fu, H., Ebert-Khosla, S., Hopwood, D. A., and Khosla, C. (1994) J. Am. Chem. Soc. 116, 4166-4170]. In addition, however, RM20b and RM20c possess two unusual features. In both molecules the hydroxyls on sp(3) C-9 and C-7 of the first six-membered ring, which arise as a result of ketoreduction and aldol condensation, respectively, are intact, rather than being lost via dehydration. Furthermore, the relative yield of RM20b (in which these hydroxyls are syn) is 7-fold greater than that of RM20c (in which they are anti). The novelty of RM20b and RM20c is indicative of the catalytic flexibility of recombinant PKSs and has encouraging implications for the generation of novel polyketides through the manipulation of gene structure and organization.