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

The Agrobacterium tumefaciens C58 genome harbors an operon containing the dmeR (Atu0890) and dmeF (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. The dmeRF 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 represses dmeRF transcription through direct binding to the promoter region upstream of dmeR A strain lacking dmeF 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 a Nicotiana 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 gene sodBII Furthermore, control of iron homeostasis via RirA is important for the ability of A. tumefaciens to cope with cobalt and nickel toxicity. The molecular mechanism of the regulation of dmeRF transcription by DmeR was demonstrated. This work provides evidence of a direct interaction of apo-DmeR with the corresponding DNA operator site in vitro The recognition site for apo-DmeR consists of 10-bp AT-rich inverted repeats separated by six C bases (5'-ATATAGTATACCCCCCTATAGTATAT-3'). Cobalt and nickel cause DmeR to dissociate from the dmeRF promoter, which leads to expression of the metal efflux gene dmeF This work also revealed a connection between iron homeostasis and cobalt/nickel resistance in A. tumefaciens.