In vivo synthesis of csypyrone derivatives by exploring the substrate diversity of start units of type Ⅲ polyketide synthase CsyB

Abstract

As a novel fungal type Ⅲ polyketide synthase, CsyB from Aspergillus oryzae can sequentially accept one molecular short chain fatty acyl CoA as start unit, one molecular malonyl-CoA and one molecular acetoacetyl-CoA as extend unit to produce the short chain csypyrone B1-3. On the basis of crystal structure of CsyB, a fatty acyl CoA binding tunnel of a length of about 16 Å is located in its active center that is proposed to accept diversified start units. In order to examine the substrate diversity of CsyB, CsyB gene was introduced and expressed in Escherichia coli that contained a number of precursors of long chain fatty acyl CoA in vivo. The results of HPLC revealed that a series of long chain csypyrone derivatives were detected in the recombinant strain in comparison with the control strain. These new csypyrone compounds were preliminarily analyzed by UV-visible spectroscopy and LC-HRMS. Three hydroxylated csypyrones were intensively determined by 1D and 2D NMR experiments, especially the position of the hydroxyl group in these compounds. These results demonstrate that CsyB exhibits a broad substrate specificity, which not only can accept the long chain saturated or unsaturated fatty acyl CoA as substrate, but also accept hydroxylated long chain fatty acyl CoA.