Reciprocal Control of Motility and Biofilm Formation by the PdhS2 Two-Component Sensor Kinase ofAgrobacterium tumefaciens

ABSTRACT A core regulatory pathway that directs developmental transitions and cellular asymmetries in Agrobacterium tumefaciens involves two overlapping, integrated phosphorelays. One of these phosphorelays putatively includes four histidine sensor kinase homologues, DivJ, PleC, PdhS1, and PdhS2, and two response regulators, DivK and PleD. In several different alphaproteobacteria, this pathway influences a conserved downstream phosphorelay that ultimately controls the phosphorylation state of the CtrA master response regulator. The PdhS2 sensor kinase reciprocally regulates biofilm formation and swimming motility. In the current study the mechanisms by which the A. tumefaciens sensor kinase PdhS2 directs this regulation are delineated. PdhS2 lacking a key residue for phosphatase activity is markedly deficient in proper control of attachment and motility phenotypes, whereas a kinase-deficient PdhS2 mutant is only modestly affected. A genetic interaction between DivK and PdhS2 is revealed, unmasking one of several connections between PdhS2-dependent phenotypes and transcriptional control by CtrA. Epistasis experiments suggest that PdhS2 can function independently of the CckA sensor kinase, the cognate sensor kinase for CtrA which is inhibited by DivK. PdhS2 dynamically localizes to the daughter cell pole in dividing cells. Global expression analysis of the pdhS2 mutant reveals a restricted regulon, functioning through CtrA to separately control motility and regulate levels of the intracellular signal cyclic diguanylate monophosphate (cdGMP), thereby affecting production of adhesive polysaccharides and attachment. We hypothesize that in A. tumefaciens the CtrA regulatory circuit has expanded to include additional inputs through addition of PdhS-type sensor kinases, likely fine-tuning the response of this organism to the soil microenvironment. IMPORTANCE Bacterial developmental processes, such as morphological transformations and behavioral transitions, are tightly regulated. In many alphaproteobacteria cell division and development are coordinated by a specific suite of conserved histidine kinases and their partnered regulatory proteins. Here we describe how the histidine kinase PdhS2 of Agrobacterium tumefaciens regulates complex phenotypes including biofilm formation and motility. PdhS2 genetically interacts with a single-domain response regulator, DivK, and the intracellular signal cyclic diguanylate monophosphate. PdhS2 dynamically localizes to the new pole of recently divided cells, contributing to the regulatory processes that dictate whether these cells remain motile or initiate biofilm formation. These findings expand our understanding of the complex network that integrates cell division and developmental control in A. tumefaciens and related alphaproteobacteria.