Proteome analysis of an Escherichia coli ptsN-null strain under different nitrogen regimes.

The carbohydrate-uptake phosphorelay PTS system plays a key role in metabolic regulation in Bacteria controlling the utilization of secondary carbon sources. Some bacteria, such as Escherichia coli, encode a paralogous system named PTSNtr (nitrogen related PTS). PTSNtr is composed of EINtr (ptsP), NPr (ptsO), and EIIANtr (ptsN). These proteins act as a phosphorelay system from phosphoenolpyruvate to EINtr, NPr and them to EIIANtr. PTSNtr is not involved in carbohydrate uptake and it may be dedicated to performing regulatory functions. The phosphorylation state of EINtr is regulated by allosteric binding of glutamine and 2-oxoglutarate, metabolites whose intracellular levels reflect the nitrogen status. Although PTSNtr is designated as having nitrogen-sensory properties, no major effect of this system on nitrogen regulation has been described in E. coli. Here we show that an E. coli ptsN deletion mutant has impaired growth in minimal medium. Proteome analysis of the ∆ptsN strain under different nitrogen regimes revealed no involvement in regulation of the canonical nitrogen regulatory (Ntr) system. The proteomic data support the conclusion that ptsN is required to balance the activities of the sigma factors RpoS and RpoD in such way that, in the absence of ptsN, RpoS-dependent genes are preferentially expressed. SIGNIFICANCE: The nitrogen related PTSNtr phosphorelay system has been hypothesized to participate in the control of nitrogen metabolism. Here we used a proteomics approach to show that an Escherichia coli ptsN null strain, which misses the final module of PTSNtr phosphorelay, has no significant effects on nitrogen metabolism under different nitrogen regimes. We noted that ptsN is required for fitness under minimal medium and for the proper balance between RpoS and sigma 70 activities in such way that, in the absence of ptsN, RpoS-dependent genes are preferentially expressed.