The plasticity of cyanobacterial metabolism supports direct CO2 conversion to ethylene

The cyanobacterial tricarboxylic acid (TCA) cycle functions in both in biosynthesis and energy generation. However, it has until recently been generally considered to be incomplete(1,2) with limited flux(3,4), and few attempts have been made to draw carbon from the cycle for biotechnological purposes. We demonstrated that ethylene can be sustainably and efficiently produced from the TCA cycle of the recombinant cyanobacterium Synechocystis 6803 expressing the Pseudomonas ethyleneforming enzyme (Efe)(5). A new strain with a modified ribosome binding site upstream of the efe gene diverts 10% of fixed carbon to ethylene and shows increased photosynthetic activities. The highest specific ethylene production rate reached 718 +/- 19 mu l l(-1) h(-1) per A(730 nm). Experimental and computational analyses based on kinetic C-13-isotope tracer and liquid chromatography coupled with mass spectrometry (LC-MS) demonstrated that the TCA metabolism is activated by the ethylene forming reaction, resulting in a predominantly cyclic architecture. The outcome significantly enhanced flux through the remodelled TCA cycle (37% of total fixed carbon) compared with a complete, but bifurcated and low-flux (13% of total fixed carbon) TCA cycle in the wild type. Global carbon flux is redirected towards the engineered ethylene pathway. The remarkable metabolic network plasticity of this cyanobacterium is manifested by the enhancement of photosynthetic activity and redistribution of carbon flux, enabling efficient ethylene production from the TCA cycle.