P
II
signal transduction proteins are widely spread among all domains of life where they regulate a multitude of carbon and nitrogen metabolism related processes. Non-diazotrophic cyanobacteria can utilize a high variety of organic and inorganic nitrogen sources. In recent years, several physiological studies indicated an involvement of the cyanobacterial P
II
protein in regulation of ammonium, nitrate/nitrite, and cyanate uptake. However, direct interaction of P
II
has not been demonstrated so far. In this study, we used biochemical, molecular genetic and physiological approaches to demonstrate that P
II
regulates all relevant nitrogen uptake systems in
Synechocystis
sp. strain PCC 6803: P
II
controls ammonium uptake by interacting with the Amt1 ammonium permease, probably similar to the known regulation of
E. coli
ammonium permease AmtB by the P
II
homolog GlnK. We could further clarify that P
II
mediates the ammonium- and dark-induced inhibition of nitrate uptake by interacting with the NrtC and NrtD subunits of the nitrate/nitrite transporter NrtABCD. We further identified the ABC-type urea transporter UrtABCDE as novel P
II
target. P
II
interacts with the UrtE subunit without involving the standard interaction surface of P
II
interactions. The deregulation of urea uptake in a P
II
deletion mutant causes ammonium excretion when urea is provided as nitrogen source. Furthermore, the urea hydrolyzing urease enzyme complex appears to be coupled to urea uptake. Overall, this study underlines the great importance of the P
II
signal transduction protein in the regulation of nitrogen utilization in cyanobacteria.
Nitrogen starvation induces developmental transitions in cyanobacteria. Whereas complex multicellular cyanobacteria of the order Nostocales can differentiate specialized cells that perform nitrogen fixation in the presence of oxygenic photosynthesis, non-diazotrophic unicellular strains, such as <i>Synechococcus elongatus</i> or <i>Synechocystis</i> PCC 6803, undergo a transition into a dormant non-growing state. Due to loss of pigments during this acclimation, the process is termed chlorosis. Cells maintain viability in this state for prolonged periods of time, until they encounter a useable nitrogen source, which triggers a highly coordinated awakening process, termed resuscitation. The minimal set of cellular activity that maintains the viability of cells during chlorosis and ensures efficient resuscitation represents the organism’s equivalent of the BIOS, the basic input/output system of a computer, that helps “booting” the operation system after switching on. This review summarizes the recent research in the resuscitation of cyanobacteria, representing a powerful model for the awakening of dormant bacteria.
The reactions of α-D-phosphohexomutases (αPHM) are ubiquitous, key to primary metabolism and essential for several processes in all domains of life. The functionality of these enzymes relies on an initial auto-phosphorylation step which requires the presence of α-D-glucose-1,6-bisphosphate (Glc-1,6-BP). While well investigated in vertebrates, the origin of this activator compound in bacteria is unknown. Here we show that the Slr1334 protein from the unicellular cyanobacterium Synechocysitis sp. PCC 6803 is a Glc-1,6-BP-synthase. Biochemical analysis revealed that Slr1334 efficiently converts fructose-1,6-bisphosphate and α-D-glucose-1-phosphate/α-D-glucose-6-phosphate into Glc-1,6-BP and also catalyzes the reverse reaction. Phylogenetic analysis revealed that the slr1334 product belongs to a primordial subfamily of αPHMs that is present especially in deeply branching bacteria and also includes human commensals and pathogens. Interestingly, the homologue of Slr1334 in the human gut bacterium Bacteroides salyersiae catalyzes the same reaction, suggesting a conserved and essential role for the members of this αPHM subfamily.
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