The regulatory role of a novel transcription factor YjjQ in LuxR family in biofilm formation, flagellar biosynthesis and virulence in Salmonella Typhimurium

Salmonella Typhimurium (S. Typhimurium) is a major foodborne Gram-negative pathogen, presenting significant health risks to both humans and animals. LuxR family regulatory factors are critically involved in bacterial adaption to environmental changes and virulence regulation. Nevertheless, the biological function of YjjQ, a member of this family, remains uncharacterized in S. Typhimurium. To clarify the regulatory function of YjjQ in S. Typhimurium, λ-Red homologous recombination was employed to construct the yjjQ gene deletion strain (SL1344-ΔyjjQ) and its complementation strain (SL1344-ΔyjjQ/comp). Then, the growth characteristics, biofilm formation, motility, cell adhesion and invasion, intracellular survival and proliferation, as well as virulence, were systematically analyzed, respectively. Furthermore, the expression profiles of those genes associated with biofilm, flagella, and virulence were evaluated, and the interaction between YjjQ and predicted target promoter was experimentally validated. Compared to SL1344 and SL1344-ΔyjjQ/comp, SL1344-ΔyjjQ exhibited notably enhanced biofilm formation, motility, cell invasion, and intracellular proliferation under hyperosmotic conditions, in addition to heightened virulence in mice. RT-qPCR analysis revealed these genes related to flagellar (flhD, fliA), biofilm (csgD, fliC), and SPI-1 (invF) were significantly upregulated. The EMSA confirmed that YjjQ protein directly binds the flhDC promoter region, suppressing transcription of the flhDC operon, thereby negatively regulating flagellar, biofilm and virulence. These findings provide new insights into the molecular mechanisms underlying the coordinately modulated strategy of biofilm formation, flagellar biosynthesis and virulence in S. Typhimurium.