A specific role of the Saccharopolyspora erythraea thioesterase II gene in the function of modular polyketide synthases

Abstract

Bacterial modular polyketide synthase (PKS) genes are commonly associated with another gene that encodes a thioesterase II (TEII) believed to remove aberrantly loaded substrates from the PKS. Co-expression of the Saccharopolyspora erythraea ery-ORF5 TEII and eryA genes encoding 6-deoxyerythronolide B synthase (DEBS) in Streptomyces hosts eliminated or significantly lowered production of 8,8′-deoxyoleandolide [15-nor-6-deoxyerythronolide B (15-nor-6dEB)], which arises from an acetate instead of a propionate starter unit. Disruption of the TEII gene in an industrial Sac. erythraea strain caused a notable amount of 15-norerythromycins to be produced by utilization of an acetate instead of a propionate starter unit and also resulted in moderately lowered production of erythromycin compared with the amount produced by the parental strain. A similar behaviour of the TEII gene was observed in Escherichia coli strains that produce 6dEB and 15-methyl-6dEB. Direct biochemical analysis showed that the ery-ORF5 TEII enzyme favours hydrolysis of acetyl groups bound to the loading acyl carrier protein domain (ACPL) of DEBS. These results point to a clear role of the TEII enzyme, i.e. removal of a specific type of acyl group from the ACPL domain of the DEBS1 loading module.