Characterizing and Engineering Trehalose Biosynthesis Pathways to Improve Acetate Tolerance in Pseudomonas putida KT2440
Alkaline pretreatment can depolymerize lignin into a diverse mixture of monomers and smaller compounds, enabling bioconversion of these compounds to value-added chemicals in engineered bacterial hosts such as Pseudomonas putida KT2440. Acetate and salts comprise approximately 8% and 32% of the lignin stream, respectively. High concentrations of these components can cause cell death in P. putida, so lignin-derived streams must be fed in lower amounts to decrease stress. Investigation of the mechanisms of bacterial tolerance to chemicals in lignin-rich substrates is therefore required to overcome this obstacle in lignin bioconversion. Previous work has shown that trehalose, an endogenously produced disaccharide, is involved in tolerance to heat, cold, and osmotic stress, as well as implicated in acid stress in other bacteria. As such, trehalose biosynthesis presents a useful target to increase acetate and osmotic tolerance in P. putida. This work investigates the native TreSA, TreSB, and TreY/TreZ trehalose biosynthesis pathways in this bacterial strain as well as the OtsAB pathway of Sphingobium sp. SYK-6. Overexpression of the OtsAB pathway in P. putida increased tolerance to acetate relative to wild-type. Additionally, functional knockouts of treSA, treSB, treY, and treZ in P. putida showed that interruption of any trehalose biosynthesis pathway, and, in the case of TreY/TreZ, interruption of the pathway at any point, significantly hindered growth in acetate as well as on glucose and LB broth. Each trehalose biosynthesis pathway was then engineered for overexpression in P. putida to determine whether overexpression of these genes conferred additional tolerance to acetate. The TreSA, TreSB, OtsA, and OtsB enzymes were also expressed and purified to determine the reaction rates and kinetics of these pathways. The hypothetical glycoside hydrolase family 15 protein, encoded in the gene located in between otsB and otsA in the Sphingobium operon, was purified as well to investigate its potential involvement in the OtsAB pathway. The findings of this work illuminated the mechanisms of trehalose biosynthesis in P. putida and identified genetic targets to improve strain tolerance to common components of alkaline-pretreated lignin streams. These findings could ultimately improve yields of desired products from lignin in engineered strains of P. putida.