Regulation of global metabolism by PTS-ntr in rhizobium leguminosarum

Thesis

Maintaining intracellular carbon-nitrogen balance is essential for rhizobial growth in the free-living state and for establishing effective symbiosis with host legumes. In these organisms, this balance is modulated post-translationally by the nitrogen-related phosphotransferase system (PTSNtr), which comprises the sensor protein PtsP (EINtr), the phosphotransfer protein NPr, and the output regulators PtsN (EIIANtr) and ManX (EIIAMan). PtsP reacts to the intracellular nitrogen status of the cell. Under nitrogen-limiting conditions, PtsP autophosphorylates at a histidine residue and subsequently transfers the phosphate to NPr, which in turn phosphorylates the output proteins PtsN and ManX. In contrast, when the cell is nitrogen-sufficient, allosteric binding of glutamine inhibits the autophosphorylation of PtsP, resulting in nonphosphorylated NPr, PtsN and ManX. The bifunctional HPr kinase/phosphorylase (HPrK/P) adds an additional layer of control by phosphorylating a conserved serine on NPr, preventing histidine phosphorylation and blocking the phosphotransfer to the downstream effectors PtsN and ManX. In this thesis, I investigated the role of PTSNtr in the metabolism of Rhizobium leguminosarum. First, the components, signalling mechanisms, and known targets of the Alphaproteobacterial PTSNtr were reviewed. The involvement of ManX and HPrK/P in carbon metabolism was then examined; mutations in these proteins were found to perturb the TCA cycle, causing carbon to be redirected to storage compounds, a response also observed under nitrogen starvation. PTSNtr was also shown to sense nitrogen availability and signal to the RelA/SpoT homolog (RSH), which regulates amino-acid uptake through the AapJQMP transport system via modulation of the AbcR1/2 sRNAs. Finally, the rhizobial homolog of the Escherichia coli phosphohistidine phosphatase SixA was characterised and shown not to affect PTSNtr function under the conditions tested.

Authors: Tjahjono, O

• DOI: 10.5287/ora-zg9z28db1
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: exopolysaccharides (EPS); (p)ppGpp; polyhydroxybutyrate (PHB); carbon metabolism; glycogen