Herbaspirillum seropedicae signal transduction protein PII is structurally similar to the enteric GlnK

The Importance of Weakly Co-Evolving Residue Networks in Proteins is Revealed by Visual Analytics

Small changes in a protein’s core packing produce changes in function, and even small changes in function bias species fitness and survival. Therefore individually deleterious mutations should be evolutionarily coupled with compensating mutations that recover fitness. Co-evolving pairs of mutations should be littered across evolutionary history. Despite longstanding intuition, the results of co-evolution analyses have largely disappointed expectations. Regardless of the statistics applied, only a small majority of the most strongly co-evolving residues are typically found to be in contact, and much of the “meaning” of observed co-evolution has been opaque. In a medium-sized protein of 300 amino acids, there are almost 20 million potentially-important interdependencies. It is impossible to understand this data in textual format without extreme summarization or truncation. And, due to summarization and truncation, it is impossible to identify most patterns in the data. We developed a visualization approach that eschews the common “look at a long list of statistics” approach and instead enables the user to literally look at all of the co-evolution statistics simultaneously. Users of our tool reported visually obvious “clouds” of co-evolution statistics forming distinct patterns in the data, and analysis demonstrated that these clouds had structural relevance. To determine whether this phenomenon generalized, we repeated this experiment in three proteins we had not previously studied. The results provide evidence about how structural constrains have impacted co-evolution, why previous “examine the most frequently co-evolving residues” approaches have had limited success, and additionally shed light on the biophysical importance of different types of co-evolution.

Native IM-Orbitrap MS: Resolving What Was Hidden

Native ion mobility-mass spectrometry (IM-MS) is an emerging biophysical approach to probe the intricate details of protein structure and function. The instrument design enables measurements of accurate first-principle determinations of rotationally-averaged ion-neutral collision cross sections coupled with high-mass, high-resolution mass measurement capabilities of Orbitrap MS. The inherent duty-cycle mismatch between drift tube IM and Orbitrap MS is alleviated by operating the drift tube in a frequency modulated mode while continuously acquiring mass spectra with the Orbitrap MS. Fourier transform of the resulting time-domain signal, i.e., ion abundances as a function of the modulation frequency, yields a frequency domain spectrum that is then converted (s-1 to s) to IM drift time. This multiplexed approach allows for a duty-cycle of 25% compared to <1% for traditional "pulse-and-wait" IM-ToF-MS. Improvements in mobility and mass resolution of the IM-Orbitrap allows for accurate analysis of intact protein complexes and the possibility of capturing protein dynamics.

Crystal structures of the apo and ATP bound Mycobacterium tuberculosis nitrogen regulatory PII protein

PII constitutes a family of signal transduction proteins that act as nitrogen sensors in microorganisms and plants. Mycobacterium tuberculosis (Mtb) has a single homologue of PII whose precise role has as yet not been explored. We have solved the crystal structures of the Mtb PII protein in its apo and ATP bound forms to 1.4 and 2.4 A resolutions, respectively. The protein forms a trimeric assembly in the crystal lattice and folds similarly to the other PII family proteins. The Mtb PII:ATP binary complex structure reveals three ATP molecules per trimer, each bound between the base of the T-loop of one subunit and the C-loop of the neighboring subunit. In contrast to the apo structure, at least one subunit of the binary complex structure contains a completely ordered T-loop indicating that ATP binding plays a role in orienting this loop region towards target proteins like the ammonium transporter, AmtB. Arg38 of the T-loop makes direct contact with the gamma-phosphate of the ATP molecule replacing the Mg(2+) position seen in the Methanococcus jannaschii GlnK1 structure. The C-loop of a neighboring subunit encloses the other side of the ATP molecule, placing the GlnK specific C-terminal 3(10) helix in the vicinity. Homology modeling studies with the E. coli GlnK:AmtB complex reveal that Mtb PII could form a complex similar to the complex in E. coli. The structural conservation and operon organization suggests that the Mtb PII gene encodes for a GlnK protein and might play a key role in the nitrogen regulatory pathway.

A novel signal transduction protein P(II) variant from Synechococcus elongatus PCC 7942 indicates a two-step process for NAGK-P(II) complex formation

P(II) signal transduction proteins are highly conserved in bacteria, archaea and plants and have key functions in coordination of central metabolism by integrating signals from the carbon, nitrogen and energy status of the cell. In the cyanobacterium Synechococcus elongatus PCC 7942, P(II) binds ATP and 2-oxoglutarate (2-OG) in a synergistic manner, with the ATP binding sites also accepting ADP. Depending on its effector molecule binding status, P(II) (from this cyanobacterium and other oxygenic phototrophs) complexes and regulates the arginine-controlled enzyme of the cyclic ornithine pathway, N-acetyl-l-glutamate kinase (NAGK), to control arginine biosynthesis. To gain deeper insights into the process of P(II) binding to NAGK, we searched for P(II) variants with altered binding characteristics and found P(II) variants I86N and I86T to be able to bind to an NAGK variant (R233A) that was previously shown to be unable to bind wild-type P(II) protein. Analysis of interactions between these P(II) variants and wild-type NAGK as well as with the NAGK R233A variant suggested that the P(II) I86N variant was a superactive NAGK binder. To reveal the structural basis of this property, we solved the crystal structure of the P(II) I86N variant at atomic resolution. The large T-loop, which prevails in most receptor interactions of P(II) proteins, is present in a tightly bended conformation that mimics the T-loop of S. elongatus P(II) after having latched onto NAGK. Moreover, both P(II) I86 variants display a specific defect in 2-OG binding, implying a role of residue I86 in 2-OG binding. We propose a two-step model for the mechanism of P(II)-NAGK complex formation: in an initiating step, a contact between R233 of NAGK and E85 of P(II) initiates the bending of the extended T-loop of P(II), followed by a second step, where a bended T-loop deeply inserts into the NAGK clefts to form the tight complex.

Expression and molecular characterization of the Mycobacterium tuberculosis PII protein

The signal transduction protein PII plays an important role in cellular nitrogen assimilation and regulation. The molecular characteristics of the Mycobacterium tuberculosis PII (Mtb PII) were investigated using biophysical experiments. The Mtb PII coding ORF Rv2919c was cloned and expressed in Escherichia coli. The binding characteristics of the purified protein with ATP and ADP were investigated using surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). Mtb PII binds to ATP strongly with K(d) in the range 1.93-6.44 microM. This binding strength was not significantly affected by the presence of 2-ketoglutarate even in molar concentrations of 66 (ITC) or 636 (SPR) fold excess of protein concentration. However, an additional enthalpy of 0.3 kcal/mol was released in presence of 2-ketoglutarate. Binding of Mtb PII to ADP was weaker by an order of magnitude. Binding of ATP and 2-ketoglutarate were analysed by docking studies on the Mtb PII crystal structure (PDB id 3BZQ). We observed that hydrogen bonds involving the gamma-phosphate of ATP contribute to enhanced binding of ATP compared with ADP. Glutaraldehyde crosslinking showed that Mtb PII exists in homotrimeric state which is consistent with other PII proteins. Phylogenetic analysis showed that Mtb PII consistently grouped with other actinobacterial PII proteins.

Effect of protein solution components in the adsorption of Herbaspirillum seropedicae GlnB protein on mica

The adsorption of proteins and its buffer solution on mica surfaces was investigated by atomic force microscopy (AFM). Different salt concentration of the Herbaspirillum seropedicae GlnB protein (GlnB-Hs) solution deposited on mica was investigated. This protein is a globular, soluble homotrimer (36kDa), member of PII-like proteins family involved in signal transducing in prokaryote. Supramolecular structures were formed when this protein was deposited onto bare mica surface. The topographic AFM images of the GlnB-Hs films showed that at high salt concentration the supramolecular structures are spherical-like, instead of the typical doughnut-like shape for low salt concentration. AFM images of NaCl and Tris from the buffer solution showed structures with the same pattern as those observed for high salt protein solution, misleading the image interpretation. XPS experiments showed that GlnB protein film covers the mica surface without chemical reaction.

Ternary complex formation between AmtB, GlnZ and the nitrogenase regulatory enzyme DraG reveals a novel facet of nitrogen regulation in bacteria

Ammonium movement across biological membranes is facilitated by a class of ubiquitous channel proteins from the Amt/Rh family. Amt proteins have also been implicated in cellular responses to ammonium availability in many organisms. Ammonium sensing by Amt in bacteria is mediated by complex formation with cytosolic proteins of the P(II) family. In this study we have characterized in vitro complex formation between the AmtB and P(II) proteins (GlnB and GlnZ) from the diazotrophic plant-associative bacterium Azospirillum brasilense. AmtB-P(II) complex formation only occurred in the presence of adenine nucleotides and was sensitive to 2-oxoglutarate when Mg(2+) and ATP were present, but not when ATP was substituted by ADP. We have also shown in vitro complex formation between GlnZ and the nitrogenase regulatory enzyme DraG, which was stimulated by ADP. The stoichiometry of this complex was 1:1 (DraG monomer : GlnZ trimer). We have previously reported that in vivo high levels of extracellular ammonium cause DraG to be sequestered to the cell membrane in an AmtB and GlnZ-dependent manner. We now report the reconstitution of a ternary complex involving AmtB, GlnZ and DraG in vitro. Sequestration of a regulatory protein by the membrane-bound AmtB-P(II) complex defines a new regulatory role for Amt proteins in Prokaryotes.

The Amt/Mep/Rh family of ammonium transport proteins

The Amt/Mep/Rh family of integral membrane proteins comprises ammonium transporters of bacteria, archaea and eukarya, as well as the Rhesus proteins found in animals. They play a central role in the uptake of reduced nitrogen for biosynthetic purposes, in energy metabolism, or in renal excretion. Recent structural information on two prokaryotic Amt proteins has significantly contributed to our understanding of this class, but basic questions concerning the transport mechanism and the nature of the transported substrate, NH3 or [NH4(+)], remain to be answered. Here we review functional and structural studies on Amt proteins and discuss the bioenergetic issues raised by the various mechanistic proposals present in the literature.

Genetic screen for regulatory mutations in Methanococcus maripaludis and its use in identification of induction-deficient mutants of the euryarchaeal repressor NrpR

NrpR is an euryarchaeal transcriptional repressor of nitrogen assimilation genes. Previous studies with Methanococcus maripaludis demonstrated that NrpR binds to palindromic operator sequences, blocking transcription initiation. The metabolite 2-oxoglutarate, an indicator of cellular nitrogen deficiency, induces transcription by lowering the affinity of NrpR for operator DNA. In this report we build on existing genetic tools for M. maripaludis to develop a screen for change-of-function mutations in a transcriptional regulator and demonstrate the use of an X-Gal (5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside) screen for strict anaerobes. We use the approach to address the primary structural requirements for the response of NrpR to 2-oxoglutarate. nrpR genes from the mesophilic M. maripaludis and the hyperthermophilic Methanopyrus kandleri were targeted for mutagenesis. M. maripaludis nrpR encodes a protein with two homologous NrpR domains while the M. kandleri nrpR homolog encodes a single NrpR domain. Random point mutagenesis and alanine replacement mutagenesis identified two amino acid residues of M. kandleri NrpR involved in induction of gene expression under nitrogen-deficient conditions and thus in the response to 2-oxoglutarate. Mutagenesis of the corresponding regions in either domain of M. maripaludis NrpR resulted in a similar effect, demonstrating a conserved structure-function relationship between the two repressors. The results indicate that in M. maripaludis, both NrpR domains participate in the 2-oxoglutarate response. The approach used here has wide adaptability to other regulatory systems in methanogenic Archaea and other strict anaerobes.

Crystal structure of Arabidopsis PII reveals novel structural elements unique to plants

The 1.9 A resolution crystal structure of PII from Arabidopsis thaliana reveals for the first time the molecular structure of a widely conserved regulator of carbon and nitrogen metabolism from a eukaryote. The structure provides a framework for understanding the arrangement of highly conserved residues shared with PII proteins from bacteria, archaea, and red algae as well as residues conserved only in plant PII. Most strikingly, a highly conserved segment at the N-terminus that is found only in plant PII forms numerous interactions with the alpha2 helix and projects from the surface of the homotrimer opposite to that occupied by the T-loop. In addition, solvent-exposed residues near the T-loop are highly conserved in plants but differ in prokaryotes. Several residues at the C-terminus that are also highly conserved only in plants contribute part of the ATP-binding site and likely participate in an ATP-induced conformational change. Structures of PII also reveal how citrate and malonate bind near the triphosphate binding site occupied by ATP in bacterial and archaeal PII proteins.

The crystal structure of the Escherichia coli AmtB-GlnK complex reveals how GlnK regulates the ammonia channel

Amt proteins are ubiquitous channels for the conduction of ammonia in archaea, eubacteria, fungi, and plants. In Escherichia coli, previous studies have indicated that binding of the PII signal transduction protein GlnK to the ammonia channel AmtB regulates the channel thereby controlling ammonium influx in response to the intracellular nitrogen status. Here, we describe the crystal structure of the complex between AmtB and GlnK at a resolution of 2.5 A. This structure of PII in a complex with one of its targets reveals physiologically relevant conformations of both AmtB and GlnK. GlnK interacts with AmtB almost exclusively via a long surface loop containing Y51 (T-loop), the tip of which inserts deeply into the cytoplasmic pore exit, blocking ammonia conduction. Y51 of GlnK is also buried in the pore exit, explaining why uridylylation of this residue prevents complex formation.

Structure of GlnK1 with bound effectors indicates regulatory mechanism for ammonia uptake

A binary complex of the ammonia channel Amt1 from Methanococcus jannaschii and its cognate P(II) signalling protein GlnK1 has been produced and characterized. Complex formation is prevented specifically by the effector molecules Mg-ATP and 2-ketoglutarate. Single-particle electron microscopy of the complex shows that GlnK1 binds on the cytoplasmic side of Amt1. Three high-resolution X-ray structures of GlnK1 indicate that the functionally important T-loop has an extended, flexible conformation in the absence of Mg-ATP, but assumes a compact, tightly folded conformation upon Mg-ATP binding, which in turn creates a 2-ketoglutarate-binding site. We propose a regulatory mechanism by which nitrogen uptake is controlled by the binding of both effector molecules to GlnK1. At normal effector levels, a 2-ketoglutarate molecule binding at the apex of the compact T-loop would prevent complex formation, ensuring uninhibited ammonia uptake. At low levels of Mg-ATP, the extended loops would seal the ammonia channels in the complex. Binding of both effector molecules to P(II) signalling proteins may thus represent an effective feedback mechanism for regulating ammonium uptake through the membrane.

Inhibitory complex of the transmembrane ammonia channel, AmtB, and the cytosolic regulatory protein, GlnK, at 1.96 A

Ammonia conductance is highly regulated. A P(II) signal transduction protein, GlnK, is the final regulator of transmembrane ammonia conductance by the ammonia channel AmtB in Escherichia coli. The complex formed between AmtB and inhibitory GlnK at 1.96-A resolution shows that the trimeric channel is blocked directly by GlnK and how, in response to intracellular nitrogen status, the ability of GlnK to block the channel is regulated by uridylylation/deuridylylation at Y51. ATP and Mg(2+) augment the interaction of GlnK. The hydrolyzed product, adenosine 5'-diphosphate orients the surface of GlnK for AmtB blockade. 2-Oxoglutarate diminishes AmtB/GlnK association, and sites for 2-oxoglutarate are evaluated.

Interaction of the membrane-bound GlnK-AmtB complex with the master regulator of nitrogen metabolism TnrA in Bacillus subtilis

PII proteins are widespread and highly conserved signal transduction proteins occurring in bacteria, Archaea, and plants and play pivotal roles in controlling nitrogen assimilatory metabolism. This study reports on biochemical properties of the PII-homologue GlnK (originally termed NrgB) in Bacillus subtilis (BsGlnK). Like other PII proteins, the native BsGlnK protein has a trimeric structure and readily binds ATP in the absence of divalent cations, whereas 2-oxoglutarate is only weakly bound. In contrast to other PII-like proteins, Mg2+ severely affects its ATP-binding properties. BsGlnK forms a tight complex with the membrane-bound ammonium transporter AmtB (NrgA), from which it can be relieved by millimolar concentrations of ATP. Immunoprecipitation and co-localization experiments identified a novel interaction between the BsGlnK-AmtB complex and the major transcription factor of nitrogen metabolism, TnrA. In vitro in the absence of ATP, TnrA is completely tethered to membrane (AmtB)-bound GlnK, whereas in extracts from BsGlnK- or AmtB-deficient cells, TnrA is entirely soluble. The presence of 4 mm ATP leads to concomitant solubilization of BsGlnK and TnrA. This ATP-dependent membrane re-localization of TnrA by BsGlnK/AmtB may present a novel mechanism to control the global nitrogen-responsive transcription regulator TnrA in B. subtilis under certain physiological conditions.

In vitro analysis of the Escherichia coli AmtB-GlnK complex reveals a stoichiometric interaction and sensitivity to ATP and 2-oxoglutarate

In Escherichia coli, the ammonia channel AmtB and the P(II) signal transduction protein GlnK constitute an ammonium sensory system that effectively couples the intracellular nitrogen regulation system to external changes in ammonium availability. Binding of GlnK to AmtB apparently inactivates the channel, thereby controlling ammonium influx in response to the intracellular nitrogen status. We designed an N-terminally histidine-tagged version of AmtB with a native C-terminal region in order to purify the AmtB-GlnK complex. Purification revealed a stable and direct interaction between AmtB and GlnK, thereby showing for the first time that stability of the complex does not require other proteins. The stoichiometry of the complex was determined by two independent approaches, both of which indicated a 1:1 ratio of AmtB to GlnK. We also showed by mass spectrometry that only the fully deuridylylated form of GlnK co-purifies with AmtB. The purified complex allowed in vitro studies of dissociation and association of AmtB and GlnK. The interaction of GlnK with AmtB is dependent on ATP and is also sensitive to 2-oxoglutarate. Our in vitro data suggest that in vivo association and dissociation of the complex might not only be dependent on the uridylylation status of GlnK but may also be influenced by intracellular pools of ATP and 2-oxoglutarate.

The poor growth of Rhodospirillum rubrum mutants lacking PII proteins is due to an excess of glutamine synthetase activity

The P(II) family of proteins is found in all three domains of life and serves as a central regulator of the function of proteins involved in nitrogen metabolism, reflecting the nitrogen and carbon balance in the cell. The genetic elimination of the genes encoding these proteins typically leads to severe growth problems, but the basis of this effect has been unknown except with Escherichia coli. We have analysed a number of the suppressor mutations that correct such growth problems in Rhodospirillum rubrum mutants lacking P(II) proteins. These suppressors map to nifR3, ntrB, ntrC, amtB(1) and the glnA region and all have the common property of decreasing total activity of glutamine synthetase (GS). We also show that GS activity is very high in the poorly growing parental strains lacking P(II) proteins. Consistent with this, overexpression of GS in glnE mutants (lacking adenylyltransferase activity) also causes poor growth. All of these results strongly imply that elevated GS activity is the causative basis for the poor growth seen in R. rubrum mutants lacking P(II) and presumably in mutants of some other organisms with similar genotypes. The result underscores the importance of proper regulation of GS activity for cell growth.

Structure of the PII signal transduction protein of Neisseria meningitidis at 1.85 A resolution

The P(II) signal transduction proteins GlnB and GlnK are implicated in the regulation of nitrogen assimilation in Escherichia coli and other enteric bacteria. P(II)-like proteins are widely distributed in bacteria, archaea and plants. In contrast to other bacteria, Neisseria are limited to a single P(II) protein (NMB 1995), which shows a high level of sequence identity to GlnB and GlnK from Escherichia coli (73 and 62%, respectively). The structure of the P(II) protein from N. meningitidis (serotype B) has been solved by molecular replacement to a resolution of 1.85 A. Comparison of the structure with those of other P(II) proteins shows that the overall fold is tightly conserved across the whole population of related proteins, in particular the positions of the residues implicated in ATP binding. It is proposed that the Neisseria P(II) protein shares functions with GlnB/GlnK of enteric bacteria.

Identification of Rhodospirillum rubrum GlnB variants that are altered in their ability to interact with different targets in response to nitrogen status signals

In Rhodospirillum rubrum, NifA, the transcriptional activator for the nif genes, is posttranslationally activated only by the uridylylated form of GlnB, one of three P(II) homologs in the organism. We have used the yeast two-hybrid system to detect variants of GlnB that interact better with NifA than does wild-type GlnB. When examined for physiological effects in R. rubrum, these GlnB* variants activated NifA in the presence of NH(4)(+), which normally blocks NifA activation completely, and in the absence of GlnD, whose uridylylation of GlnB is also normally essential for NifA activation. When these variants were tested in the two-hybrid system for their interaction with NtrB, a receptor that should interact with the nonuridylylated form of GlnB, they were uniformly weaker than wild-type GlnB in that interaction. When expressed in R. rubrum either as single-copy integrants or on multiple-copy plasmids, these variants were also dramatically altered in terms of their ability to regulate several other receptors involved in nitrogen metabolism, including GlnE, NtrB/NtrC, and DRAT (dinitrogenase reductase ADP-ribosyl transferase)-DRAG (dinitrogenase reductase-activating glycohydrolase). The consistent pattern throughout is that these GlnB variants partially mimic the uridylylated form of wild-type GlnB, even under nitrogen-excess conditions and in strains lacking GlnD. The results suggest that the role of uridylylation of GlnB is primarily to shift the equilibrium of GlnB from a "nitrogen-sufficient" form to a "nitrogen-deficient" form, each of which interacts with different but overlapping receptor proteins in the cell. These GlnB variants apparently shift that equilibrium through direct structural changes.

History on the biological nitrogen fixation research in graminaceous plants: special emphasis on the Brazilian experience

This review covers the history on Biological Nitrogen Fixation (BNF) in Graminaceous plants grown in Brazil, and describes research progress made over the last 40 years, most of whichwas coordinated by Johanna Döbereiner. One notable accomplishment during this period was the discovery of several nitrogen-fixing bacteria such as the rhizospheric (Beijerinckia fluminensis and Azotobacter paspali), associative (Azospirillum lipoferum, A. brasilense, A. amazonense) and the endophytic (Herbaspirillum seropedicae, H. rubrisubalbicans, Gluconacetobacter diazotrophicus, Burkholderia brasilensis and B. tropica). The role of these diazotrophs in association with grasses, mainly with cereal plants, has been studied and a lot of progress has been achieved in the ecological, physiological, biochemical, and genetic aspects. The mechanisms of colonization and infection of the plant tissues are better understood, and the BNF contribution to the soil/plant system has been determined. Inoculation studies with diazotrophs showed that endophytic bacteria have a much higher BNF contribution potential than associative diazotrophs. In addition, it was found that the plant genotype influences the plant/bacteria association. Recent data suggest that more studies should be conducted on the endophytic association to strengthen the BNF potential. The ongoing genome sequencing programs: RIOGENE (Gluconacetobacter diazotrophicus) and GENOPAR (Herbaspirillum seropedicae) reflect the commitment to the BNF study in Brazil and should allow the country to continue in the forefront of research related to the BNF process in Graminaceous plants.

Crystal structures of the signal transducing protein GlnK from Thermus thermophilus HB8

The Thermus thermophilus HB8 genome encodes a signal transducing PII protein, GlnK. The crystal structures of GlnK have been determined in two different space groups, P2(1)2(1)2(1) and P3(1)21. The PII protein has the T-loop, which is essential for interactions with receptor proteins. In both crystal forms, three GlnK molecules form a trimer in the asymmetric unit. In one P2(1)2(1)2(1) crystal form, the three T-loops in the trimer are disordered, while in another P2(1)2(1)2(1) crystal form, the T-loop from one molecule in the trimer is ordered. In the P3(1)21 crystal, one T-loop is ordered while the other two T-loops are disordered. The conformations of the ordered T-loops significantly differ between the two crystal forms; one makes the alpha-helix in the middle of the T-loop, while the other has an extension of the beta-hairpin. Two different conformations are captured by the crystal contacts. The observation of multiple T-loop conformations suggests that the T-loop could potentially exhibit "polysterism," which would be important for interactions with receptor proteins. The crystal structures of the nucleotide-bound forms, GlnK.ATP and GlnK.ADP, have also been determined. ATP/ADP binding within a cleft at the interface of two adjacent T. thermophilus GlnK monomers might affect the conformation of the T-loop.

Effect of T- and C-loop mutations on the Herbaspirillum seropedicae GlnB protein in nitrogen signalling

Proteins of the PII family are found in species of all kingdoms. Although these proteins usually share high identity, their functions are specific to the different organisms. Comparison of structural data from Escherichia coli GlnB and GlnK and Herbaspirillum seropedicae GlnB showed that the T-loop and C-terminus were variable regions. To evaluate the role of these regions in signal transduction by the H. seropedicae GlnB protein, four mutants were constructed: Y51F, G108A/P109a, G108W and Q3R/T5A. The activities of the native and mutated proteins were assayed in an E. coli background constitutively expressing the Klebsiella pneumoniae nifLA operon. The results suggested that the T-loop and C-terminus regions of H. seropedicae GlnB are involved in nitrogen signal transduction.

Global carbon/nitrogen control by PII signal transduction in cyanobacteria: from signals to targets

PII signal transduction plays a pervasive role in microbial nitrogen control. Different phylogenetic lineages have developed various signal transduction schemes around the highly conserved core of the signalling system, which consists of the PII proteins. Among all various bacterial PII signalling systems, the one in cyanobacteria is so far unique: in unicellular strains, the mode of covalent modification is by serine phosphorylation and the interpretation of the cellular nitrogen status occurs by measuring the 2-oxoglutarate levels. Recent advances have been the identification of the phospho-PII phosphatase, the resolution of the crystal structure of PII proteins from Synechococcus and Synechocystis strains and the identification of novel functions of PII regulation in cyanobacteria, which highlight the central role of PII signalling for the acclimation to changing carbon-nitrogen regimes.