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

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 ⌬irr mutant under iron-limiting conditions, but reversed to wild-type levels in an ⌬irr ⌬rirA mutant. The level of uncomplexed iron in the ⌬irr mutant was decreased, whereas the ⌬rirA mutant exhibited elevated iron levels. Sensitivity of the ⌬irr and ⌬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 ⌬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 ⌬rirA mutant. In the ⌬irr ⌬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.

Roles of Agrobacterium tumefaciens membrane-bound ferritin (MbfA) in iron transport and resistance to iron under acidic conditions

“…Previous studies in Bradyrhizobium japonicum, Rhizobium leguminosarum, A. tumefaciens and Rhodobacter sphaeroides demonstrated that transcription of the mbfA gene is regulated by iron levels, and is negatively controlled by the iron response regulator (irr) under ironlimiting conditions (Hibbing & Fuqua, 2011;Peuser et al, 2012;Ruangkiattikul et al, 2012;Rudolph et al, 2006;Todd et al, 2006). It has been shown that the expression of A. tumefaciens mbfA is also inducible upon exposure to H 2 O 2 and MbfA plays a role in H 2 O 2 resistance, possibly by removing intracellular free iron and thus preventing the Fenton reaction (Ruangkiattikul et al, 2012).…”

Cysteine desulphurase-encoding gene sufS2 is required for the repressor function of RirA and oxidative resistance in Agrobacterium tumefaciens

“…Irr and RirA are regulators of the suf operon (Hibbing & Fuqua, 2011;Todd et al, 2006). The Fe-S containing the RirA form may be used as a reporter for sensing cellular Fe-S status and controlling synthesis via the suf operon in response to changing demands for Fe-S clusters.…”

“…3). Irr and RirA are involved in iron regulation (Hibbing & Fuqua, 2011;Todd et al, 2006) and also in the induction of the suf operon by oxidative stress (Fig. 3).…”

“…Moreover, ironresponsive regulation of the suf operon by an additional unknown mechanism was observed (Hibbing & Fuqua, 2011).…”

“…It has been reported that the A. tumefaciens suf operon is regulated by the iron response regulator (Irr) and the rhizobial iron regulator (RirA) under low iron and high iron conditions, respectively (Hibbing & Fuqua, 2011). Moreover, ironresponsive regulation of the suf operon by an additional unknown mechanism was observed (Hibbing & Fuqua, 2011).…”