Phosphatase to kinase switch of a critical enzyme contributes to timing of cell differentiation

Cell differentiation is an essential biological process that is often subject to strict temporal regulation. The aquatic bacterium, Caulobacter crescentus, undergoes obligate differentiation from a swarmer cell to a stationary, replication-competent stalked cell, with each cell cycle. Here, we report that the switch from phosphatase to kinase activity of the histidine kinase PleC contributes to timing this differentiation event. We show that PleC PAS domain interaction with the polar scaffold protein PodJ localizes PleC to the cell pole and inhibits in vivo kinase activity. Upon PodJ degradation, released PleC switches to its kinase form and phosphorylates the PleD diguanylate cyclase, initiating the signaling pathway responsible for differentiation. While PodJ inhibits PleC kinase activity, it does not impact PleC phosphatase activity on DivK, which is required for pili biogenesis and flagellar rotation. Thus, PleC PAS domain interaction with PodJ regulates PleC subcellular localization, enzymatic activity, and the timing of cell differentiation, revealing that PAS domains affect enzymatic function on diverse substrates by relying on context dependent binding partners.