The nitrogen phosphotransferase system (PTSNtr) ofPseudomonas putidais a multi-component regulatory device that participates in controlling a variety of physiological processes in a post-translational fashion. A general survey of genes regulated by PtsN exposed transcription of thekdpFABCoperon is most conspicuously affected. Measurements ofkdpFppromoter activity in differentptsmutants showed that PtsN is responsible for repression ofkdpFABCtranscription. This effect could be assigned mainly to PtsN˜P, depending on the external K+ concentration. Bacterial two-hybrid assays demonstrated thatkdpFpregulation is implemented through direct interaction of the PtsN protein with the sensor kinase KdpD of the KdpD/KdpE two-component system. Interaction between KdpD and PtsN was detectable with a PtsN variant that imitates the non-phosphorylated form as well as with a PtsN type mimicking the phosphorylated form of PtsN. These results raise a regulatory scenario in which the Kdp system is regulated by the action of PtsN through direct interaction with the sensor kinase KdpD, and the outcome of such an interaction depends on the phosphorylation state of PtsN as well as on the external K+ concentration.