The preprotein translocase YajC mediates biofilm formation and symbiotic nitrogen-fixation in Mesorhizobium huakuii

ABSTRACT In bacteria, the auxiliary complex SecDF–YajC participates in the Sec system, facilitating post-translational translocation of preproteins across the membrane through the SecYEG channel. Although YajC homologs are strongly conserved in rhizobial strains and may play important roles in many processes, the functions and mechanisms by which they are involved in the interaction between rhizobia and host legumes are unknown. By constructing Mesorhizobium huakuii yajC mutant, we observed an increase in the strain growth rate, but a reduction in both the biomass and average thickness of the biofilm in the yajC gene mutant compared to the wild type. Deletion of yajC gene results in severe suppression of competitive ability in the plant rhizosphere, alongside a substantial reduction in root hair curling and infection thread initiation during the early infection stages. The yajC-deficient mutant formed more numerous but smaller root nodules, with a 77% decrease in nitrogen-fixing capacity. Scanning electron microscopy analysis revealed that yajC mutant bacteroids displayed, alongside clear hallmarks of cellular deformation, dissociation, and premature senescence. From nodule bacteroid proteomic analysis, we further identified 210 differentially expressed proteins, including 32 transport-related proteins, and 10 associated with nitrogen fixation. Altogether, our findings reveal the importance of YajC-mediated transport and biofilm formation in rhizobial infection and nodule development during M. huakuii-Astragalus sinicus symbiosis. IMPORTANCE In Mesorhizobium huakuii-Astragalus sinicus symbiosis, deletion of the conserved Sec subunit YajC delays early biofilm formation by impairing flagellar-driven attachment. However, the yajC mutant restores wild-type biofilm biomass later, via upregulation of EPS synthesis genes ( exoA / exoY ), the PTS(Ntr) regulator ptsN , and polysaccharide deacetylase. This recovery contrasts with permanent biofilm defects in other bacteria, underscoring the need for longer-term assessment. YajC loss also disrupts nitrogenase activity by impairing iron-sulfur cluster biogenesis, Nif complex assembly, and electron transfer. As a compensatory response, plants form more nodules, offsetting reduced nitrogenase activity per nodule. Thus, multipath processes like biofilm formation show adaptability to secretion defects, whereas essential, linear pathways (e.g., nitrogenase maturation) remain highly sensitive. Our work redefines YajC: beyond its canonical role in Sec-mediated secretion, it is a rhizobium-specific factor critical for symbiosis, advancing understanding of secretion system roles in host-microbe interactions.