Abstract
TonB-ExbB-ExbD-like energy transduction systems are widespread among Gram-negative bacteria. While most species have only one copy of tonB-exbBD genes, the Pseudomonas species possess more TonB-ExbBD homologues. One of them, the TonB3-PocA-PocB complex, was recently shown to be required for polar localization of FlhF and, thus, the flagella in Pseudomonas aeruginosa Here, we show that the orthologous TonBm-PocA-PocB complex is important for polar localization of FlhF and flagella in Pseudomonas putida as well. Additionally, the system is necessary for maintaining membrane integrity, as the inactivation of the TonBm-PocAB complex results in increased membrane permeability, lowered stress tolerance, and conditional cell lysis. Interestingly, the functionality of TonBm-PocAB complex is more important for stationary than for exponentially growing bacteria. The whole-cell proteome analysis provided a likely explanation for this growth phase dependence, as extensive reprogramming was disclosed in an exponentially growing tonBm deletion strain, while only a few proteomic changes, mostly downregulation of outer membrane proteins, were determined in the stationary-phase ΔtonBm strain. We propose that this response in exponential phase, involving, inter alia, activation of AlgU and ColR regulons, can compensate for TonBm-PocAB's deficiency, while stationary-phase cells are unable to alleviate the lack of TonBm-PocAB. Our results suggest that mislocalization of flagella does not cause the membrane integrity problems; rather, the impaired membrane intactness of the TonBm-PocAB-deficient strain could be the reason for the random placement of flagella.IMPORTANCE The ubiquitous Pseudomonas species are well adapted to survive in a wide variety of environments. Their success relies on their versatile metabolic, signaling, and transport ability but also on their high intrinsic tolerance to various stress factors. This is why the study of the stress-surviving mechanisms of Pseudomonas species is of utmost importance. The stress tolerance of Pseudomonads is mainly achieved through the high barrier property of their membranes. Here, we present evidence that the TonB-ExbBD-like TonBm-PocAB system is involved in maintaining the membrane homeostasis of Pseudomonas putida, and its deficiency leads to lowered stress tolerance and conditional cell lysis.