Diverse MarR bacterial regulators of auxin catabolism in the plant microbiome.
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Demonstrated by isothermal titration calorimetry (ITC) that MarR_73 (IadR) directly binds indole-3-acetic acid (IAA) with Kd = 0.41 +- 0.03 uM and solved its 1.3 A crystal structure in complex with IAA (PDB 7KFO).
"MarR_73 bound IAA with 0.41 ± 0.03 µM affinity, while no binding to MarR_50 was detected."
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Resolved the IAA-binding pocket of IadR (MarR_73), where Tyr15, Ala18, Trp45, Arg46, and Val65 coordinate the indole ring and Ser28, His32, and Arg46 coordinate the IAA carboxylate.
"The MarR_73 residues coordinating the indole ring of IAA are Tyr-15, Ala-18, Trp-45, Arg-46 and Val-65 (Fig. 2b), while those coordinating the IAA carboxylate are Ser-28, His-32 and Arg-46 (Fig. 2c)."
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Showed by DNA-binding assays, ΔMarR_73 deletion genetics, and RNA-seq that IadR binds a 24 bp operator site between iadR and iadA to repress the iad operon in the absence of IAA and releases DNA upon IAA binding to derepress transcription.
"These data (Fig. 2) demonstrate that MarR_73 functions as a repressor of this locus: in the absence of IAA, MarR_73 binds to a DNA site between MarR_73 (iadR) and iadA to repress expression of the IAA degradation genes. When IAA is present, it binds to MarR_73, releasing MarR_73 from the DNA site to allow expression of the IAA degradation genes downstream."