Structural and functional differences between BenM and CatM, two LysR-type paralagos in Acinetobacter baylyi ADP1
Ezezika, Obidimma Chukwunonso
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BenM and CatM are two LysR-type transcriptional regulators that activate the ben and cat genes needed for benzoate degradation in the soil bacterium, Acinetobacter baylyi ADP1. Both proteins respond to a metabolite of benzoate degradation, cis,cis-muconate. In addition, BenM, but not CatM, responds to benzoate as an effector. BenM plays the primary role in expression of the benABCDE genes responsible for the initial catabolism of benzoate. CatM regulates the expression of the catBCIJFD genes needed for the subsequent degradation of cis,cis-muconate to tricarboxylic acid cycle intermediates. In the absence of BenM, CatM is not sufficient to activate the benABCDE operon to high enough levels to enable growth on benzoate. The regulation of CatM at the benABCDE promoter was studied in light of two mutations in CatM that enabled high level activation of benABCDE operon, CatM(V158M) and CatM(R156H). These substitutions affected benA and catB gene expression differently, thus highlighting the importance of balanced expression from multiple promoters in the benzoate degradation pathway. The ability of these CatM variants to substitute for BenM underscored the evolutionary relationship between both proteins. To understand further the differences between these proteins, X-ray diffraction methods were used to solve the structures of their effector binding domains with and without their effectors, muconate and benzoate bound to the protein. The structures highlighted the important features of how both proteins respond to effectors to initiate transcriptional activation and represent the first structures of the family of LysR-type transcriptional regulators with bound effectors. These investigations postulate a molecular basis for the synergism exhibited by BenM in response to both benzoate and cis,cis-muconate in which a charge relay system underlies the synergistic transcriptional activation. In light of these studies, the structures of the effector binding domains of CatM(V158M), CatM(R156H) and another BenM variant, BenM(R156H/T157S) were determined. In comparing these variant structures with their wild-type counterpart structures, the role that these substitutions play in affecting transcriptional regulation was addressed. In separate studies, two independent crystal structures of the BenM-effector binding domain generated from high pH conditions show how certain conditions favor oligomerization of the full-length protein that lead to solubility problems. A detailed model by which BenM, CatM and other LysR-type transcriptional regulators could form these oligomers is presented.