The REC domain mediated dimerization is critical for FleQ from Pseudomonas aeruginosa to function as a c-di-GMP receptor and flagella gene regulator

FleQ is an AAA+ ATPase enhancer-binding protein that regulates both flagella and biofilm formation in the opportunistic pathogen Pseudomonas aeruginosa. FleQ belongs to the NtrC subfamily of response regulators, but lacks the corresponding aspartic acid for phosphorylation in the REC domain (FleQR, also named FleQ domain). Here, we show that the atypical REC domain of FleQ is essential for the function of FleQ. Crystal structure of FleQR at 2.3 Å reveals that the structure of FleQR is significantly different from the REC domain of NtrC1 which regulates gene expression in a phosphorylation dependent manner. FleQR forms a novel active dimer (transverse dimer), and mediates the dimerization of full-length FleQ in an unusual manner. Point mutations that affect the dimerization of FleQ lead to loss of function of the protein. Moreover, a c-di-GMP binding site deviating from the previous reported one is identified through structure analysis and point mutations.