PTSNtr is a regulatory phosphotransferase system in many bacteria. Mutation of the PTSNtr enzymes causes pleiotropic growth phenotypes, dry colony morphology and a posttranslational inactivation of ABC transporters in Rhizobium leguminosarum 3841. The PTSNtr proteins EINtr and 2 copies of EIIANtr have been described previously. Here we identify the intermediate phosphocarrier protein NPr and show its phosphorylation by EINtr in vitro. Furthermore we demonstrate that phosphorylation of EINtr and NPr is required for ABC transport activation and that the N-terminal GAF domain of EINtr is not required for autophosphorylation. Previous studies have shown that non-phosphorylated EIIANtr is able to modulate the transcriptional activation of the high affinity potassium transporter KdpABC. In R. leguminosarum 3841 kdpABC expression strictly depends on EIIANtr. Here we demonstrate that under strong potassium limitation ABC transport is inactivated, presumably by non-phosphorylated EIIANtr. This is to our knowledge the first report where PTSNtr dictates an essential cellular function. This is achieved by the inverse regulation of two important ATP dependent transporter classes.