Regulation of the Cobalt/Nickel Efflux Operon dmeRF in Agrobacterium tumefaciens and a Link between the Iron-Sensing Regulator RirA and Cobalt/Nickel Resistance

TheAgrobacterium tumefaciens C58 genome harbors an operon containing thedmeR (Atu0890) anddmeF (Atu0891) genes, which encode a transcriptional regulatory protein belonging to the RcnR/CsoR family and a metal efflux protein belonging to the cation diffusion facilitator (CDF) family, respectively. ThedmeRF operon is specifically induced by cobalt and nickel, with cobalt being the more potent inducer. Promoter-lacZ transcriptional fusion, an electrophoretic mobility shift assay, and DNase I footprinting assays revealed that DmeR repressesdmeRF transcription through direct binding to the promoter region upstream ofdmeR . A strain lackingdmeF showed increased accumulation of intracellular cobalt and nickel and exhibited hypersensitivity to these metals; however, this strain displayed full virulence, comparable to that of the wild-type strain, when infecting aNicotiana benthamiana plant model under the tested conditions. Cobalt, but not nickel, increased the expression of many iron-responsive genes and reduced the induction of the SoxR-regulated genesodBII . Furthermore, control of iron homeostasis via RirA is important for the ability ofA. tumefaciens to cope with cobalt and nickel toxicity.IMPORTANCE The molecular mechanism of the regulation ofdmeRF transcription by DmeR was demonstrated. This work provides evidence of a direct interaction of apo-DmeR with the corresponding DNA operator sitein vitro . The recognition site for apo-DmeR consists of 10-bp AT-rich inverted repeats separated by six C bases (5′-ATATAGTATA TATAGTATAT -3′). Cobalt and nickel cause DmeR to dissociate from thedmeRF promoter, which leads to expression of the metal efflux genedmeF . This work also revealed a connection between iron homeostasis and cobalt/nickel resistance inA. tumefaciens .