The difference in in vivo sensitivity between Bacillus licheniformis PerR and Bacillus subtilis PerR is due to the different cellular environments

The cellular redox state in Bacillus amyloliquefaciens WH1 affects biofilm formation indirectly in a surfactant direct manner

Surfactin is a signal to trigger biofilm formation against harsh environments. Generally, harsh environments can result in change of the cellular redox state to induce biofilm formation, but we know little about whether the cellular redox state influences biofilm formation via surfactin. Here, the reductant glucose could reduce surfactin and enhance biofilm formation by a surfactin-indirect way. The oxidant H₂ O₂ led to a decrease of surfactin accompanying with weakened biofilm formation. Spx and PerR were both necessary for surfactin production and biofilm formation. H₂ O₂ improved surfactin production but inhibited biofilm formation by a surfactin-indirect manner in Δspx, while it reduced surfactin production without obvious influence on biofilm formation in ΔperR. The ability against H₂ O₂ stress was enhanced in Δspx, but weakened in ΔperR. Thereby, PerR was favorable for resisting oxidative stress, while Spx played a negative role in this action. Knockout and compensation of rex also supported that the cells could form biofilm by a surfactin-indirect way. Collectively, surfactin is not a unique signal to trigger biofilm formation, and the cellular redox state can influence biofilm formation by a surfactin-direct or -indirect way in Bacillus amyloliquefaciens WH1.

Redox Sensing by Fe2+ in Bacterial Fur Family Metalloregulators

SIGNIFICANCE

Iron is required for growth and is often redox active under cytosolic conditions. As a result of its facile redox chemistry, iron homeostasis is intricately involved with oxidative stress. Bacterial adaptation to iron limitation and oxidative stress often involves ferric uptake regulator (Fur) proteins: a diverse set of divalent cation-dependent, DNA-binding proteins that vary widely in both metal selectivity and sensitivity to metal-catalyzed oxidation. Recent Advances: Bacteria contain two Fur family metalloregulators that use ferrous iron (Fe²⁺) as their cofactor, Fur and PerR. Fur functions to regulate iron homeostasis in response to changes in intracellular levels of Fe²⁺. PerR also binds Fe²⁺, which enables metal-catalyzed protein oxidation as a mechanism for sensing hydrogen peroxide (H₂O₂).

CRITICAL ISSUES

To effectively regulate iron homeostasis, Fur has an Fe²⁺ affinity tuned to monitor the labile iron pool of the cell and may be under selective pressure to minimize iron oxidation, which would otherwise lead to an inappropriate increase in iron uptake under oxidative stress conditions. Conversely, Fe²⁺ is bound more tightly to PerR but exhibits high H₂O₂ reactivity, which enables a rapid induction of peroxide stress genes.

FUTURE DIRECTIONS

The features that determine the disparate reactivity of these proteins with oxidants are still poorly understood. A controlled, comparative analysis of the affinities of Fur/PerR proteins for their metal cofactors and their rate of reactivity with H₂O₂, combined with structure/function analyses, will be needed to define the molecular mechanisms that have facilitated this divergence of function between these two paralogous regulators.

The roles of two O-donor ligands in the Fe2+-binding and H2O2-sensing by the Fe2+-dependent H2O2 sensor PerR

PerR is a metal-dependent peroxide sensing transcription factor which controls the expression of genes involved in peroxide resistance. The function of Bacillus subtilis PerR is mainly dictated by the regulatory metal ion (Fe²⁺ or Mn²⁺) coordinated by three N-donor ligands (His37, His91, and His93) and two O-donor ligands (Asp85 and Asp104). While H₂O₂ sensing by PerR is mediated by Fe²⁺-dependent oxidation of N-donor ligand (either His37 or His91), one of the O-donor ligands (Asp104), but not Asp85, has been proposed as the key residue that regulates the sensitivity of PerR to H₂O₂. Here we systematically investigated the relative roles of two O-donor ligands of PerR in metal-binding affinity and H₂O₂ sensitivity in vivo and in vitro. Consistent with the previous report, in vitro the D104E-PerR could not sense low levels of H₂O₂ in the presence of excess Fe²⁺ sufficient for the formation of the Fe²⁺-bound D104E-PerR. However, the expression of PerR-regulated reporter fusion was not repressed by D104E-PerR in the presence of Fe²⁺, suggesting that Fe²⁺ is not an effective corepressor for this mutant protein in vivo. Furthermore, in vitro metal titration assays indicate that D104E-PerR has a significantly reduced affinity for Fe²⁺, but not for Mn²⁺, when compared to wild type PerR. These data indicate that the type of O-donor ligand (Asp vs. Glu) at position 104 is an important determinant in providing high Fe²⁺-binding affinity required for the sensing of the physiologically relevant Fe²⁺-levels, in addition to its role in rendering PerR highly sensitive to physiological levels of H₂O₂. In comparison, the D85E-PerR did not show a perturbed change in Fe²⁺-binding affinity, however, it displayed a slightly decreased sensitivity to H₂O₂ both in vivo and in vitro, suggesting that the type of O-donor ligand (Asp vs. Glu) at position 85 may be important for the fine-tuning of H₂O₂ sensitivity.

The inability of Bacillus licheniformis perR mutant to grow is mainly due to the lack of PerR-mediated fur repression

PerR, a member of Fur family protein, is a metal-dependent H₂O₂ sensing transcription factor that regulates genes involved in peroxide stress response. Industrially important bacterium Bacillus licheniformis contains three PerR-like proteins (PerR_BL, PerR2, and PerR3) compared to its close relative Bacillus subtilis. Interestingly, unlike other bacteria including B. subtilis, no authentic perR _BL null mutant could be established for B. licheniformis. Thus, we constructed a conditional perR _BL mutant using a xylose-inducible promoter, and investigated the genes under the control of PerR_BL. PerR_BL regulon genes include katA, mrgA, ahpC, pfeT, hemA, fur, and perR as observed for PerR_BS. However, there is some variation in the expression levels of fur and hemA genes between B. subtilis and B. licheniformis in the derepressed state. Furthermore, katA, mrgA, and ahpC are strongly induced, whereas the others are only weakly or not induced by H₂O₂ treatment. In contrast to the B. subtilis perR null mutant which frequently gives rise to large colony phenotype mainly due to the loss of katA, the suppressors of B. licheniformis perR mutant, which can form colonies on LB agar, were all catalase-positive. Instead, many of the suppressors showed increased levels of siderophore production, suggesting that the suppressor mutation is linked to the fur gene. Consistent with this, perR fur double mutant could grow on LB agar without Fe supplementation, whereas perR katA double mutant could only grow on LB agar with Fe supplementation. Taken together, our data suggest that in B. licheniformis, despite the similarity in PerR_BL and PerR_BS regulon genes, perR is an essential gene required for growth and that the inability of perR null mutant to grow is mainly due to elevated expression of Fur.