Identification of Domains within Megalomicin and Erythromycin Polyketide Synthase Modules Responsible for Differences in Polyketide Production Levels in Escherichia coli

Abstract

The megalomicin and erythromycin polyketide synthases (PKSs) produce the same aglycon product, 6-deoxyerythronolide B (6-dEB). Both PKSs were examined in an Escherichia coli strain metabolically engineered to support complex polyketide biosynthesis. Production of 6-dEB in shake flask fermentations was undetectable by mass spectrometry in the strain expressing the megalomicin (Meg) PKS genes, whereas 31 mg/L 6-dEB was produced by the strain with the erythromycin (DEBS) PKS. The genes for each of the three subunits comprising the PKSs were expressed in different combinations from three compatible expression vectors (e.g., DEBS1, DEBS2, and MegA3) to identify two Meg PKS subunits, MegA1 and MegA3, which conferred lower 6-dEB titers than their DEBS counterparts. Comparison of protein expression levels and 6-dEB titers by engineered hybrid DEBS/Meg PKS genes further defined regions within modules 2 and 6 of MegA1 and MegA3, respectively, which limit protein expression and 6-dEB production in E. coli. Meg module 2 + TE (M2 + TE) and a hybrid DEBS M2/Meg M2 + TE protein were engineered and purified for in vitro comparisons with DEBS M2 + TE. The specific activity of the hybrid M2 + TE was approximately 16-fold lower than DEBS M2 + TE and only twice as high as the Meg M2 + TE enzyme in diketide elongation assays. Since the hybrid M2 worked comparably to DEBS M2 in vivo, this suggests that boosting subunit concentration could serve as a useful approach to overcome enzyme deficiencies in heterologous polyketide production.