Agrobacterium tumefaciens membrane-bound ferritin plays a role in protection against hydrogen peroxide toxicity and is negatively regulated by the iron response regulator

Abstract

An Agrobacterium tumefaciens membrane-bound ferritin (mbfA) mutant was generated to assess the physiological functions of mbfA in response to iron and hydrogen peroxide (H(2) O(2) ) stresses. Wild-type and the mbfA mutant strains showed similar growth under high- and low-iron conditions. The mbfA mutant was more sensitive to H(2) O(2) than wild-type strain. Expression of a functional mbfA gene could complement the H(2) O(2) -hypersensitive phenotype of the mbfA mutant and a rhizobial iron regulator (rirA) mutant, suggesting that MbfA protects cells from H(2) O(2) toxicity by sequestering intracellular free iron, thus preventing the Fenton reaction. The expression of mbfA could be induced in response to iron and to H(2) O(2) treatment. The iron response regulator (irr) also acted as a repressor of mbfA expression. An irr mutant had high constitutive expression of mbfA, which partly contributed to the H(2) O(2) -hyperresistant phenotype of the irr mutant. The data reported here demonstrate an important role of A. tumefaciens MbfA in the cellular defence against iron and H(2) O(2) stresses.

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

“…The high induction of mbfA expression observed during exposure to iron under acidic conditions further supports the notion that A. tumefaciens mbfA is involved in protection against acid-enhanced iron toxicity. An irr mutant strain,WK074, (Ruangkiattikul et al, 2012) exhibited increased expression of mbfA 75-fold in LB 5.5 and 100-fold in LB 5.5+Fe compared with WT in LB 5.5, which is consistent with the repressor role of Irr. The fact that iron-mediated mbfA induction under acidic conditions was still observed in an irr mutant indicates that the induction was Irr independent.…”

“…It has been shown previously that mutation of the mbfA gene does not affect the growth of A. tumefaciens in LB medium (pH 7.0) under either high-iron (100 mM FeCl 3 ) or low-iron (300 mM Dy) conditions (Ruangkiattikul et al, 2012). Acidic conditions help to increase iron toxicity by stabilizing the more soluble and reactive form of iron (Fe 2+ ).…”

“…It was proposed that the increased hypersensitivity of NR114 to H 2 O 2 resulted from enhanced levels of the ironcatalysed reaction (Ruangkiattikul et al, 2012). To test if the mutant NR114 exhibited increased accumulation of iron in the reactive form, which can drive the Fenton reaction, the sensitivity to SNG was tested.…”

“…To assess the influence of mbfA on the total cellular iron content, WT NTL4 and the mbfA mutant (NR114) (Ruangkiattikul et al, 2012) were analysed for total iron content using ICP-MS. The results showed that exponentialphase cells of WT and mutant strains grown in LB medium or LB supplemented with 50 mM FeCl 3 had no obvious difference in total iron content (data not shown).…”

“…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). Here, the topology and function of A. tumefaciens MbfA were further investigated.…”