In any given organism, the function of one-third of all proteins is unknown. Some of these proteins have a domain of unknown function, such as the arginine-rich DUF1127. Approximately 20,000 proteins with a DUF1127 domain have been identified in about 5,000 bacteria. The plant pathogen Agrobacterium tumefaciens, encodes seven DUF1127 proteins: three small ones (47 and 48 amino acids) and four long ones (72 to 101 amino acids). Our current study aims at identifying the interaction partners and biological role of DUF1127 proteins in A. tumefaciens and other bacteria.

A triple A. tumefaciens mutant having all three small DUF1127 genes deleted, exhibits a variety of phenotypes, including a growth defect in stationary phase, upregulated siderophores and altered phosphate uptake. Complementation with any of the small DUF1127 genes restores the wildtype phenotypes. Moreover, DUF1127 genes from a range of bacteria including Escherichia coli, Rhodobacter sphaeroides, Rhizobium rubi, Sinorhizobium meliloti and Neorhizobium galegae can complement the phenotypes of the triple deletion mutant. Our results suggest that the function of small DUF1127 proteins is conserved across different species. One major group of upregulated genes in the A. tumefaciens triple mutant is involved in phosphate uptake and belongs to the pst operon. The transcription of this operon is regulated by the two-component system PhoB-PhoR. We provide evidence that small DUF1127 proteins interact with the sensor kinase PhoR. Given that the phosphate metabolism in an E. coli DUF1127 mutant also is mis-regulated, we hypothesize that DUF1127 proteins have a conserved function in this process that is mediated through protein-protein interactions.