Novel proteins of the phosphotransferase system encoded within the rpoN operon of Escherichia coli: Enzyme IIA^Ntr affects growth on organic nitrogen and the conditional lethality of an era^ts mutant

Two rpoN-linked ΔTn10-kan insertions suppress the conditionally lethal era^ts allele. One truncates rpoN while the second disrupts another gene (ptsN) in the rpoN operon and does not affect classical nitrogen regulation. Neither alter expression of era indicating that suppression is post-translational. Plasmid clones of ptsN prevent suppression by either disruption mutation indicating that this gene is important for lethality caused by era^ts. rpoN and six neighboring genes were sequenced and compared with sequences in the database. Two of these genes encode proteins homologous to Enzyme IIA^Fru and HPr of the phosphoenolpyruvate:sugar phosphotransferase system. We designate these proteins IIA^Ntr (ptsN) and NPr (npr). Purified IIA^Ntr and NPr exchange phosphate appropriately with Enzyme I, HPr, and Enzyme HA proteins of the phosphoenolpyruvate: sugar phosphotransferase system. Several sugars and tricarboxylic acid cycle intermediates inhibited growth of the ptsN disruption mutant on medium containing an amino acid or nucleoside base as a combined source of nitrogen, carbon, and energy. This growth inhibition was relieved by supplying the ptsN gene or ammonium salts but was not aleviated by altering levels of exogenously supplied cAMP. These results support our previous proposal of a novel mechanism linking carbon and nitrogen assimilation and relates IIA^Ntr to the unknown process regulated by the essential GTPase Era.