Oxidation of propionate in Pseudomonas sp. LFM046: Relevance to the synthesis of polyhydroxyalkanoates containing odd-chain length monomers and 2-methylisocitrate

Pseudomonas sp. LFM046 produces polyhydroxyalkanoates of medium-chain length. When carbohydrates are used, only even-chain lenght monomers (3HAeven) are generated. Propionate, when used as a co-substrate, enabled the synthesis of odd-chain length monomers, albeit with poor yields (Y3HAodd/prop.) of approx. 10 %. A mini-Tn5 mutant (LFM693) was generated by interrupting the prpB gene, which encodes the 2-methylisocitrate lyase responsible for one step in propionate oxidation via 2-methylcitrate cycle. At low propionate concentrations of 0.1-0.3 g.L-1, LFM693 showed higher Y3HAodd/prop. In contrast, the propionyl incorporation into the polymer was less efficient at high propionate concentrations. The excretion of 2-methylisocitrate partially explained this reduced efficiency. However, mass balances and 13C isotopomer analysis revealed significant propionate oxidation in LFM693 of 21 % and 46 % for propionate concentrations of 0.5 and 1.0 g.L-1, respectively. Experiments using C1-labeled (13C) propionate revealed an alternative propionate oxidation pathway (alpha-oxidation, beta-oxidation, or 3-methylmalate) whose genes are yet to be revealed.