Investigation of the PTS-Ntr system in Rhizobium leguminosarum

Investigation of the PTS-Ntr system in Rhizobium leguminosarum

The “nitrogen phospho transferase” (PTS-Ntr) system is paralogous to the PTS sugar transport systems in E. coli, which are well characterised. It consists of a phospho relay system of 3 proteins: EINtr (encoded by ptsP), Npr and EIIANtr (encoded by ptsN). Autophosphorylation of EINtr is the starting point of the phosphorylation cascade. The PTS-Ntr system is much wider spread in the bacterial world than PTS sugar transport systems, but much less well understood. Mutations in the PTS-Ntr system cause pleiotropic phenotypes in many bacteria, but functional relationships have been only recently established in E. coli (Lee et al. 2007; Lüttmann et al. 2009). The E. coli PTS-Ntr can directly interact with K+ transport systems of different affinity via unphosphorylated EIIANtr and might therefore regulate K+ homeostasis.

Mutation of ptsP encoding EINtr in Rhizobium leguminosarum strain Rlv3841 caused a pleiotropic phenotype as observed with many bacteria (Prell et al. 2012). The mutant formed dry colonies and grew poorly on organic nitrogen or dicarboxylates. Most strikingly the ptsP mutant had low activity of a broad range of ATP-dependent ABC transporters. This lack of activation, which occurred post-translationally, may explain many of the pleiotropic effects. In contrast proton coupled transport systems were not inhibited in a ptsP mutant. Regulation by PtsP also involves two copies of ptsN that code for EIIANtr, resulting in the above mentioned phosphorylation cascade. As in E. coli, the Rlv3841 PTSNtr system also regulates K+ homeostasis by transcriptional activation of the high affinity ATP dependent K+ transporter KdpABC. This involves direct interaction of a two component sensor regulator pair KdpDE with unphosphorylated EIIANtr. Critically, ptsP mutants, which cannot phosphorylate PtsN1 or PtsN2, had a fully activated KdpABC transporter. This is the opposite pattern from that observed with ABC transporters which apparently require phosphorylation of PtsN. These results suggest that ATP-dependent transport might be regulated via PTS-Ntr responding to the cellular energy charge. ABC transport may be inactivated at low energy charge, conserving ATP for essential processes including K+ homeostasis.

Lee CR, Cho SH, Yoon MJ, Peterkofsky A and Seok YJ (2007) Escherichia coli enzyme IIANtr regulates the K+ transporter TrkA. Proc. Natl. Acad. Sci. USA 104: 4124-4129.

Luttmann D, Heermann R, Zimmer B, Hillmann A, Rampp IS, Jung K and Gorke B, (2009) Stimulation of the potassium sensor KdpD kinase activity by interaction with the phosphotransferase protein IIANtr in Escherichia coli. Mol. Microbiol. 72: 978-994.

Prell J, Mulley G, Haufe F, White JP, Williams A, Karunakaran R, Downie JA and Poole PS (2012) The PTSNtr system globally regulates ATP dependent transporters in Rhizobium leguminosarum. Mol. Microbiol. accepted