Function of the N-terminal region of the phosphate-sensing histidine kinase, SphS, in Synechocystis sp. PCC 6803

Nitrogen availability facilitates phosphorus acquisition by bloom-forming cyanobacteria

Cyanobacterial blooms are threatening freshwater ecosystems. The physiological basis involved in the onset of cyanobacterial bloom is fundamental to advance in bloom predictions. Generally, cyanobacteria grow until the availability of nitrogen (N), phosphorus (P) or both nutrients becomes limited. Population survival may depend on physiological adjustments to nutrient deficiency as well as on the efficient use of episodic N and P inputs. This study investigated the effect of N inputs on phosphate uptake affinity and activity of N-deficient bloom-forming cyanobacteria. Lake samples dominated by filamentous cyanobacteria were preincubated with and without nitrate addition, and the uptake of [32P] phosphate pulses was measured in the following days. Phosphate uptake kinetics were analyzed with a flow-force model that provides the threshold concentration, reflecting phosphate uptake affinity, and the membrane conductivity coefficient that corresponds to the activity of uptake systems. After 24 h of nitrate preincubation, phosphate uptake kinetics showed a progressive increase in affinity (nanomolar [Pe]A) and activity (25-fold) concomitant with cyanobacterial growth. It was demonstrated that the alleviation of N-deficiency by N inputs boosts the activation of phosphate uptake systems of non-N2-fixing cyanobacteria to sustain growth. Therefore, reduction of dissolved inorganic N levels in lakes is also mandatory to limit cyanobacterial phosphate uptake affinity and activity capabilities.

PhoU Allows Rapid Adaptation to High Phosphate Concentrations by Modulating PstSCAB Transport Rate in Sinorhizobium meliloti

Maintenance of cellular phosphate homeostasis is essential for cellular life. The PhoU protein has emerged as a key regulator of this process in bacteria, and it is suggested to modulate phosphate import by PstSCAB and control activation of the phosphate limitation response by the PhoR-PhoB two-component system. However, a proper understanding of PhoU has remained elusive due to numerous complications of mutating phoU, including loss of viability and the genetic instability of the mutants. Here, we developed two sets of strains of Sinorhizobium meliloti that overcame these limitations and allowed a more detailed and comprehensive analysis of the biological and molecular activities of PhoU. The data showed that phoU cannot be deleted in the presence of phosphate unless PstSCAB is inactivated also. However, phoU deletions were readily recovered in phosphate-free media, and characterization of these mutants revealed that addition of phosphate to the environment resulted in toxic levels of PstSCAB-mediated phosphate accumulation. Phosphate uptake experiments indicated that PhoU significantly decreased the PstSCAB transport rate specifically in phosphate-replete cells but not in phosphate-starved cells and that PhoU could rapidly respond to elevated environmental phosphate concentrations and decrease the PstSCAB transport rate. Site-directed mutagenesis results suggested that the ability of PhoU to respond to phosphate levels was independent of the conformation of the PstSCAB transporter. Additionally, PhoU-PhoU and PhoU-PhoR interactions were detected using a bacterial two-hybrid screen. We propose that PhoU modulates PstSCAB and PhoR-PhoB in response to local, internal fluctuations in phosphate concentrations resulting from PstSCAB-mediated phosphate import.IMPORTANCE Correct maintenance of cellular phosphate homeostasis is critical in all kingdoms of life and in bacteria involves the PhoU protein. This work provides novel insights into the role of the Sinorhizobium meliloti PhoU protein, which plays a key role in rapid adaptation to elevated phosphate concentrations. It is shown that PhoU rapidly responds to elevated phosphate levels by significantly decreasing the phosphate transport of PstSCAB, thereby preventing phosphate toxicity and cell death. Additionally, a new model for phosphate sensing in bacterial species which involves the PhoR-PhoB two-component system is presented. This work provides new insights into the bacterial response to changing environmental conditions and into regulation of the phosphate limitation response that influences numerous bacterial processes, including antibiotic production and virulence.

Comparative analysis of kdp and ktr mutants reveals distinct roles of the potassium transporters in the model cyanobacterium Synechocystis sp. strain PCC 6803

Photoautotrophic bacteria have developed mechanisms to maintain K(+) homeostasis under conditions of changing ionic concentrations in the environment. Synechocystis sp. strain PCC 6803 contains genes encoding a well-characterized Ktr-type K(+) uptake transporter (Ktr) and a putative ATP-dependent transporter specific for K(+) (Kdp). The contributions of each of these K(+) transport systems to cellular K(+) homeostasis have not yet been defined conclusively. To verify the functionality of Kdp, kdp genes were expressed in Escherichia coli, where Kdp conferred K(+) uptake, albeit with lower rates than were conferred by Ktr. An on-chip microfluidic device enabled monitoring of the biphasic initial volume recovery of single Synechocystis cells after hyperosmotic shock. Here, Ktr functioned as the primary K(+) uptake system during the first recovery phase, whereas Kdp did not contribute significantly. The expression of the kdp operon in Synechocystis was induced by extracellular K(+) depletion. Correspondingly, Kdp-mediated K(+) uptake supported Synechocystis cell growth with trace amounts of external potassium. This induction of kdp expression depended on two adjacent genes, hik20 and rre19, encoding a putative two-component system. The circadian expression of kdp and ktr peaked at subjective dawn, which may support the acquisition of K(+) required for the regular diurnal photosynthetic metabolism. These results indicate that Kdp contributes to the maintenance of a basal intracellular K(+) concentration under conditions of limited K(+) in natural environments, whereas Ktr mediates fast potassium movements in the presence of greater K(+) availability. Through their distinct activities, both Ktr and Kdp coordinate the responses of Synechocystis to changes in K(+) levels under fluctuating environmental conditions.

Functional analysis of the N-terminal region of an essential histidine kinase, Hik2, in the cyanobacterium Synechocystis sp. PCC 6803

Histidine kinases are sensory proteins involved in the perception of environmental changes. Here, we characterized one of three essential histidine kinases, Hik2, in the cyanobacterium Synechocystis sp. PCC 6803 by constructing a fused sensor, Hik2n-Hik7c, which has the signal input domain of Hik2 and the kinase domain of the phosphate-deficiency sensor Hik7. The coding region of the hik7 gene was replaced with the fused sensor to evaluate the signalling activity in vivo as the activity of alkaline phosphatase (AP), which is regulated by Hik7. Cells expressing Hik2n-Hik7c had weak AP activities under standard growth conditions. Saline stress by NaCl induced AP activity in a dose-dependent manner. Analysis of the effects of several salt compounds on induction of AP activity indicated that Hik2n-Hik7c responded to Cl^- concentration. Amino acid substitution in the signal input domain of Hik2 resulted in loss of this responsiveness. These results suggest that the signal input domain of Hik2 responds to environmental Cl^- concentration in Synechocystis.

Molecular cloning, characterization and differential expression of DRK1 in Sporothrix schenckii

The dimorphism of Sporothrix schenckii (S. schenckii) reflects a developmental switch in morphology and lifestyle that is necessary for virulence. DRK1, a hybrid histidine kinase, functions as a global regulator of dimorphism and virulence in Blastomyces dermatitidis (B. dermatitidis) and Histoplasma capsulatum (H. capsulatum). The partial cDNA sequence of DRK1 of S. schenckii, designated SsDRK1, was obtained using degenerate primers based on the conserved domain of the DRK1 of other fungi. The complete cDNA sequence of SsDRK1 was obtained by 5' and 3' RACE. The full-length cDNA is 4743 bp in size and has an open reading frame (ORF) of 4071 bp, encoding 1356 amino acid residues. The predicted molecular mass of SsDRK1 is 147.3 kDa with an estimated theoretical isoelectric point of 5.46. The deduced amino acid sequence of SsDRK1 shows 65% identity to that of B. dermatitidis. The SsDRK1 was predicted to be a soluble histidine kinase and to contain three parts: sensor domain, linker domain and functional domain. Quantitative real-time RT-PCR revealed that SsDRK1 was more highly expressed in the yeast stage compared with that in the mycelial stage, which indicated that the SsDRK1 may be involved in the dimorphic switch in S. schenckii.

Na+-stimulated phosphate uptake system in Synechocystis sp. PCC 6803 with Pst1 as a main transporter

Background

Most living cells uptake phosphate, an indispensable nutrient for growth from their natural environment. In Synechocystis sp. PCC 6803, the cells lack phosphate-inorganic transport (Pit) system but contain two phosphate-specific transport (Pst) systems, Pst1 and Pst2. We investigated the kinetics of Pi uptake of these two Pst systems by constructing the two mutants, ΔPst1 and ΔPst2, and comparing their kinetic properties with those of the wild-type cells under both Pi-sufficient and deficient conditions. The effects of pH and Na⁺ on the uptake of phosphate in Synechocystis were also studied.

Results

Growth rates of the two mutants and wild type were similar either under phosphate-sufficient or deficient condition. The K_m for phosphate uptake was 6.09 μM in wild type and this was reduced to 0.13 μM in ΔPst1 cells and 5.16 μM in the ΔPst2 strain. The V_max values of 2.48, 0.22, and 2.17 μmol • (min • mg of chlorophyll a)^-1 were obtained for wild type, the ΔPst1 and ΔPst2 strains, respectively. A monophasic phosphate uptake was observed in wild-type cells. The uptake of phosphate was energy and pH-dependent with a broad pH optimum between pH 7-10. Osmolality imposed by NaCl stimulated phosphate uptake whereas that imposed by sorbitol decreased uptake, suggesting stimulation of uptake was dependent upon ionic effects.

Conclusion

The data demonstrate that Pst2 system of Synechocystis has higher affinity toward phosphate with lower V_max than Pst1 system. The Pst1 system had similar K_m and V_max values to those of the wild type suggesting that Pst1 is the main phosphate transporter in Synechocystis sp. PCC 6803. The K_m of Pst1 of Synechocystis is closer to that of Pit system than to that of the Pst system of E. coli, suggesting that Synechocystis Pst1 is rather a medium/low affinity transporter whereas Pst2 is a high affinity transporter.