Abstract
CO 2 is fixed by all living organisms with an autotrophic metabolism, among which the Calvin-Benson-Bassham ( CBB) cycle is the most important and widespread carbon fixation pathway. Thus, studying and engineering the CBB cycle with the associated energy providing pathways to increase the CO 2 fixation efficiency of cells is an important subject of biological research with significant application potential. In this work, the autotrophic microbe Ralstonia eutropha H16 was selected as a research platform for CBB cycle optimization engineering. By knocking out either CBB operon genes on the operon or mega-plasmid of R. eutropha , we found that both CBB operons were active and contributed almost equally to the carbon fixation process. With similar knock-out experiments, we found while both soluble and membrane-bound hydrogenases (SH and MBH), belonging to the energy providing hydrogenase module, were f unctional d uring autotrophic growth of R. eutropha. And SH played a more significant role. By introducing a heterologous cyanobacterial RuBisCO with the endogenous GroES/EL chaperone system and RbcX, the culture OD 600 of engineered strain increased 89.15% after 72 hours of autotrophic growth, indicating cyanobacterial RuBisCO with a higher activity was functional in R. eutropha and improved upon original CBB pathway. Meanwhile, expression of hydrogenases were optimized by modulating the expression of MBH and SH, which could further increase the R. eutropha H16 culture OD 600 to 93.4% at 72 hours. Moreover, the autotrophic yield of its major industrially relevant product, polyhydroxybutyrate (PHB), was increased by 99.71%. To our best knowledge, this is the first report of successfully engineering the CBB pathway of R. eutropha for improved activity , and is one of only a few cases where the efficiency of CO 2 assimilation pathway was improved. Our work demonstrates that R. eutropha is an extremely useful platform for studying and engineering the CBB for applications in more important organisms, such as agricultural crops, and a potential microbial cell factory to develop industrial biotechnology for sequestrating CO 2 .
Evidence for the Novel Metabolic Pathway of
Ralstonia eutropha
H16 to Metabolize L‐ascorbate Ryan Joyce, Tyler Stack, Michael Carter
