Enhancing Secretion of Endoglucanase in Zymomonas mobilis by Disturbing Peptidoglycan Synthesis

Zymomonas mobilis (Z. mobilis) is a potential candidate strain for consolidated bioprocessing (CBP) in lignocellulosic biorefinery. However, the low-level secretion of cellulases limits this CBP process, and the mechanism of protein secretion that is affected by cell wall peptidoglycan is also not well understood. Here, we constructed several penicillin-binding protein (PBP)-deficient strains derived from Z. mobilis S192 to perturb the cell wall peptidoglycan network and then investigated the effects of peptidoglycan on the endoglucanase secretion. The results showed that extracellular recombinant endoglucanase production was significantly enhanced in PBP mutant strains, notably, Δ1089/0959 (4.09-fold) and Δ0959 (5.76-fold) in comparison to parent strains. For PBP-deficient strains, the growth performance was not significantly inhibited, but cell morphology was altered. In addition, enhanced antibiotic sensitivity and reduced inhibitor tolerance were also detected in our study. The concentration of intracellular soluble peptidoglycan was increased, especially for single-gene deletion. Outer membrane permeability of PBP-deficient strains was also improved, notably, Δ1089/0959 (1.14-fold) and Δ0959 (1.07-fold), which might explain the increased endoglucanase extracellular secretion. Our findings indicated that PBP-deficient Z. mobilis was capable of increasing endoglucanase extracellular secretion via cell wall peptidoglycan disturbance, and it will provide a foundation for the development of CBP technology in Z. mobilis in the future. IMPORTANCE Cell wall peptidoglycan has the function to maintain cell robustness and acts as the barrier to secret recombinant proteins from the cytoplasm to extracellular space in Z. mobilis and other bacteria. Herein, we perturbed the peptidoglycan synthesis network via knocking out PBPs (ZMO0197, ZMO0959, ZMO1089) to enhance recombinant endoglycanase extracellular secretion in Z. mobilis S912. This study could lay the foundation for understanding the regulatory network of cell wall synthesis and guide the construction of CBP strains in Z. mobilis.