Small DUF1127 proteins regulate bacterial phosphate metabolism through protein–protein interactions with the sensor kinase PhoR

The domain of unknown function 1127 (DUF1127) is widely distributed among bacteria, often in proteins shorter than 50 amino acids. In the plant pathogen <i>Agrobacterium tumefaciens</i>, the absence of three small DUF1127 proteins leads to a range of phenotypic changes. In this study, we investigated the role of these small DUFs in phosphate acquisition. Upregulation of phosphate transport systems in the triple mutant resulted in increased phosphate uptake, polyphosphate accumulation, and growth defects. Using Far-Western dot blots, pulldown experiments, and the bacterial two-hybrid system, we identified a direct interaction between the small DUFs and the sensor kinase PhoR, which regulates phosphate metabolism together with the response regulator PhoB. Complementation studies revealed that DUF1127 proteins from <i>Sinorhizobium meliloti, Rhodobacter sphaeroides</i>, and <i>Escherichia coli</i> could restore the phenotypes in the <i>A. tumefaciens</i> triple mutant. Notably, an <i>E. coli</i> mutant lacking YjiS, the sole DUF1127 protein in this species, showed upregulated expression of phosphate uptake genes and accelerated phosphate uptake. Furthermore, we provide evidence for an interaction between YjiS and <i>E. coli</i> PhoR, suggesting that DUF1127-containing proteins may share a conserved regulatory function across different bacterial species. These findings provide new insights into the function of small DUF1127 proteins, demonstrating that they can act through protein-protein interactions.