Detection and Omega-Functionalization of Free Fatty Acids Produced by the Cyanobacterium Synechocystis sp. PCC 6803

In this work, secretion of free fatty acids (FFAs) and ω-hydroxy FFAs wasachieved in the model cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis), and FFAs were detected by a novel fluorescence assay. Current methods of detecting FFA concentrations, including HPLC-based and GC-based methods or enzyme-based kits, have hindered research advancement due to their laborious and/or expensive nature. The work herein establishes a novel, rapid, fluorescence-based assay for detecting total FFA concentrations secreted by Synechocystis FFA secretion strains. The novel FFA-detection assay demonstrates the efficacy of using Nile Red as a fluorescent reporter for laurate or palmitate at concentrations up to 500 µM in the presence of cationic surfactants. Total FFA concentrations in Synechocystis supernatants quantified by the novel, Nile Red fluorescence-based assay are demonstrated herein to be highly correlative to total FFA concentrations quantified by LC-MS; this correlation was seen in supernatant samples of wild type Synechocystis and Synechocystis FFA secretion strains, both in 96-well plates and 30-mL, aerated culture tubes. This work also establishes the expression of a cytochrome P450 fusion enzyme, CYP153A-CPRmut, or a monooxygenase system from Pseudomonas putida GPo1, AlkBGT, in FFA secretion strains of Synechocystis for the generation of ω-hydroxy laurate from laurate. After finding greatly increased ω-hydroxylation activity of CYP153A-CPRmut with concurrent superoxide dismutase and catalase overexpression, 55 or 1.5 µM of ω-hydroxy laurate were produced over five days by Synechocystis strains expressing CYP153A-CPRmut or AlkBGT, respectively. As further indication of the presence of reactive oxygen species affecting ω-hydroxy laurate production with Synechocystis strains expressing CYP153A-CPRmut, concentrations of ω-hydroxy laurate in the supernatant increased over two-fold in the presence of 250 µM of the anti-oxidant, methionine, in bench-scale cultures and in 96-well plate cultures. Additionally, a mutation at the 55th amino acid position in AlkB (tryptophan to cysteine; AlkBW55C), resulted in a more than two-fold shift in AlkB’s substrate preference from decanoate towards the desired substrate, laurate. As a result, Synechocystis expressing AlkBW55C could produce 5.9 µM ω-hydroxy laurate and 2.0 µM dodecanedioic acid over five days of growth.