Contact with solid surfaces activates signaling pathways that promote biofilm formation in many bacteria. The alphaproteobacterium Caulobacter crescentus uses its flagellum to sense surfaces and responds by synthesizing an adhesive called the holdfast. The C. crescentus surface sensing pathway can be activated by mutating genes required for the assembly of the flagellum or genes required for chemotaxis. However, flagellar assembly and chemotaxis mutations activate distinct surface sensing pathways that differ in the activation of the diguanylate cyclase PleD. Here, we used a genome-wide screen to identify cmrA (CCNA_02061) as a crucial determinant of hyperadhesion in the chemotaxis mutant {Delta}cheYII. Genetic analysis showed that cmrA is important for activation of PleD in a context-specific manner. It is dispensable in wild-type and late-stage flagellar ({Delta}flgH) mutant backgrounds but promotes adhesion in early-stage flagellar assembly ({Delta}fliF), chemotaxis ({Delta}cheYII) and stator ({Delta}motB) mutant backgrounds. Fluorescently tagged CmrA displays a mostly cytoplasmic localization in genetic backgrounds where cmrA is dispensable for adhesion but localizes to the cell pole in backgrounds where it regulates adhesion. Structural modeling indicates that CmrA is a degenerate, catalytically inactive GGDEF/EAL domain containing protein, but cmrA alleles with mutated conserved c-di-GMP coordinating residues are unable to support hyperadhesion. Our results indicate that altering the directional switching of MotAB stators recruits CmrA to the cell pole where it activates PleD to drive surface adaptation. Ultimately, this work underscores the complexity of flagellar surface sensing by highlighting how the many rotational states of the motor stimulate distinct but overlapping c-di-GMP signaling pathways.
Salemi, R. I., Hershey, D. M.
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