ActJK Signal Transduction System Contributes to Copper and Zinc Resistance in Sinorhizobium meliloti

Two-component systems (TCS) play a vital role in bacterial adaptation to environmental stresses, including acid and metal toxicity. The ActJK system has been previously identified as a key regulator of acid tolerance in Sinorhizobium meliloti, a model organism for studying rhizobia-legume symbioses. Here, we expand the functional scope of ActJK, demonstrating its involvement in responses to copper and zinc stress. Using transcriptional fusion assays, we observed the ActJK's operon activation through degP1's upstream promoter region induction by Cu²⁺ and Zn²⁺ in a dose-dependent manner, a response absent in the ΔactJ mutant. Comparative proteomic analysis revealed distinct yet related overlapping regulatory pathways under Cu²⁺ and Zn²⁺ stress, with differentially expressed proteins suggesting convergence in efflux pump regulation and stress response mechanisms. The ΔactJ mutant displayed reduced tolerance to metal stress, impaired expression of metal detoxification genes, and altered antibiotic susceptibility profiles. Together, these findings highlight the intricate interplay between metal resistance and multidrug efflux systems in S. meliloti and underscore ActJK as a vital regulator integrating responses to environmental fluctuations. Understanding these regulatory mechanisms enhances our knowledge of bacterial survival in soil and rhizosphere under abiotic stress conditions and offers potential applications for optimising agricultural inoculants for acidic and metal-contaminated soils.