PII proteins as global regulators of bacterial nitrogen metabolism
Abstract: Nitrogen is an essential element to sustain life, being a component of most biological macromolecules. In spite of the abundance of gaseous N2, the availability of nitrogen compounds that can be readily used by most microorganisms is scarce and its production energetically demanding. Due to the central importance of nitrogen metabolism, most microorganisms evolved elaborate mechanisms to ensure efficient regulation, balancing substrate availability, product formation and energy expenditure.In most bacteria, many archaea and some plants, the different aspects of nitrogen metabolism are coordinated by members of the PII family of signal transduction proteins, acting as fundamental molecular messengers controlling several cellular processes. In proteobacteria, including the nitrogen fixing organism Rhodospirillum rubrum, these proteins are involved in regulation at different levels: they regulate gene expression, modulating the activity of several transcription factors; they control the flux through the ammonium transport protein (AmtB); they influence the activity of key metabolic enzymes, e.g. glutamine synthetase (GS) and nitrogenase. The signal sensing and integration by these proteins is achieved in two different yet interdependent strategies: allosteric regulation (by the binding of metabolites like ATP, ADP, 2-oxoglutarate) and reversible post-translational modification. Signal integration likely results in different conformations of the proteins, influencing the direct protein-protein interaction with the cellular targets.In the present work, using R. rubrum as a model organism, we have studied some aspects of the biochemistry of PII proteins in terms of regulatory interactions with the ammonium transport protein AmtB1 and the adenylyltransferase GlnE (involved in GS regulation). Additionally, we have investigated the post-translational modification of PII proteins, showing for the first time in vivo in addition in vitro selectivity in the modification of different PII proteins.Our results contributed to elucidate several new aspects in the regulation by PII proteins and also strengthened the idea that these proteins act as global regulators in the context of bacterial nitrogen metabolism.
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