Antiparallel and Interlinked Control of Cellular Iron Levels by the Irr and RirA Regulators of Agrobacterium tumefaciens

Abstract

ABSTRACT The plant pathogen Agrobacterium tumefaciens encodes predicted iron-responsive regulators, Irr and RirA, that function in several other bacteria to control the response to environmental iron levels. Deletion mutations of irr and rirA , alone and in combination, were evaluated for their impact on cellular iron response. Growth was severely diminished in the Δ irr mutant under iron-limiting conditions, but reversed to wild-type levels in an Δ irr Δ rirA mutant. The level of uncomplexed iron in the Δ irr mutant was decreased, whereas the Δ rirA mutant exhibited elevated iron levels. Sensitivity of the Δ irr and Δ rirA mutants to iron-activated antimicrobial compounds generally reflected their uncomplexed-iron levels. Expression of genes that encode iron uptake systems was decreased in the Δ irr mutant, whereas that of iron utilization genes was increased. Irr function required a trihistidine repeat likely to mediate interactions with heme. Iron uptake genes were derepressed in the Δ rirA mutant. In the Δ irr Δ rirA mutant, iron uptake and utilization genes were derepressed, roughly combining the phenotypes of the single mutants. Siderophore production was elevated in the rirA mutant, but most strongly regulated by an RirA-controlled sigma factor. Expression of rirA itself was regulated by Irr, RirA, and iron availability, in contrast to irr expression, which was relatively stable in the different mutants. These studies suggest that in A. tumefaciens , the Irr protein is most active under low-iron conditions, inhibiting iron utilization and activating iron acquisition, while the RirA protein is active under high-iron conditions, repressing iron uptake.