Post-translational modification of heterologously expressed Streptomyces type II polyketide synthase acyl carrier proteins

Abstract

Expression in Escherichia coli of Streptomgces acyl carrier proteins (ACPs) associated with polyketide biosynthesis using the pT7-7 expression system of Tabor and Richardson led to the production predominantly of inactive ape-proteins lacking the 4'-phosphopantetheinyl prosthetic group essential for polyketide synthase activity. Modification of growth conditions led to an increase of production of active holo-protein for the actinorhodin (act) ACP, but this technique was ineffective for oxytetracycline (otc) and griseusin (gris) ACPs. Labelling experiments revealed that a low level of otc ACP expressed prior to induction was produced mainly as active holo-protein, while post-induction 15N-labelled protein was almost exclusively in the apo-ACPP form. Limiting endogenous holo-acyl carrier protein synthase (ACPS) concentration was implicated as responsible for low apo-ACP to holo-ACP conversion, rather than limiting substrate (coenzyme A) and cofactor (Mg2+) concentrations. Co-expression of act and gris ACPs with ACPS in E. coli led to high levels of production of active holo-ACPs and ACPS. We have also made the significant observation that AGES is able to transfer acylated CoA moieties to act apo-ACP.