CdbA is a DNA-binding protein and c-di-GMP receptor important for nucleoid organization and segregation in Myxococcus xanthus

Regulatory roles of the second messenger c-di-GMP in beneficial plant-bacteria interactions

The rhizosphere system of plants hosts a diverse consortium of bacteria that confer beneficial effects on plant, such as plant growth-promoting rhizobacteria (PGPR), biocontrol agents with disease-suppression activities, and symbiotic nitrogen fixing bacteria with the formation of root nodule. Efficient colonization in planta is of fundamental importance for promoting of these beneficial activities. However, the process of root colonization is complex, consisting of multiple stages, including chemotaxis, adhesion, aggregation, and biofilm formation. The secondary messenger, c-di-GMP (cyclic bis-(3'-5') dimeric guanosine monophosphate), plays a key regulatory role in a variety of physiological processes. This paper reviews recent progress on the actions of c-di-GMP in plant beneficial bacteria, with a specific focus on its role in chemotaxis, biofilm formation, and nodulation.

EvfG is a multi-function protein located in the Type VI secretion system for ExPEC

The Type VI secretion system (T6SS) functions as a protein transport nanoweapon in several stages of bacterial life. Even though bacterial competition is the primary function of T6SS, different bacteria exhibit significant variations. Particularly in Extraintestinal pathogenic Escherichia coli (ExPEC), research into T6SS remains relatively limited. This study identified the uncharacterized gene evfG within the T6SS cluster of ExPEC RS218. Through our experiments, we showed that evfG is involved in T6SS expression in ExPEC RS218. We also found evfG can modulate T6SS activity by competitively binding to c-di-GMP, leading to a reduction in the inhibitory effect. Furthermore, we found that evfG can recruit sodA to alleviate oxidative stress. The research shown evfG controls an array of traits, both directly and indirectly, through transcriptome and additional tests. These traits include cell adhesion, invasion, motility, drug resistance, and pathogenicity of microorganisms. Overall, we contend that evfG serves as a multi-functional regulator for the T6SS and several crucial activities. This forms the basis for the advancement of T6SS function research, as well as new opportunities for vaccine and medication development.

Borrelia burgdorferi PlzA is a cyclic-di-GMP dependent DNA and RNA binding protein

The PilZ domain-containing protein, PlzA, is the only known cyclic di-GMP binding protein encoded by all Lyme disease spirochetes. PlzA has been implicated in the regulation of many borrelial processes, but the effector mechanism of PlzA was not previously known. Here we report that PlzA can bind DNA and RNA and that nucleic acid binding requires c-di-GMP, with the affinity of PlzA for nucleic acids increasing as concentrations of c-di-GMP were increased. A mutant PlzA that is incapable of binding c-di-GMP did not bind to any tested nucleic acids. We also determined that PlzA interacts predominantly with the major groove of DNA and that sequence length plays a role in DNA binding affinity. PlzA is a dual-domain protein with a PilZ-like N-terminal domain linked to a canonical C-terminal PilZ domain. Dissection of the domains demonstrated that the separated N-terminal domain bound nucleic acids independently of c-di-GMP. The C-terminal domain, which includes the c-di-GMP binding motifs, did not bind nucleic acids under any tested conditions. Our data are supported by computational docking, which predicts that c-di-GMP binding at the C-terminal domain stabilizes the overall protein structure and facilitates PlzA-DNA interactions via residues in the N-terminal domain. Based on our data, we propose that levels of c-di-GMP during the various stages of the enzootic life cycle direct PlzA binding to regulatory targets.

Functional diversity of c-di-GMP receptors in prokaryotic and eukaryotic systems

Cyclic bis-(3', 5')-dimeric guanosine monophosphate (c-di-GMP) is ubiquitous in many bacterial species, where it functions as a nucleotide-based secondary messenger and is a vital regulator of numerous biological processes. Due to its ubiquity, most bacterial species possess a wide range of downstream receptors that has a binding affinity to c-di-GMP and elicit output responses. In eukaryotes, several enzymes and riboswitches operate as receptors that interact with c-di-GMP and transduce cellular or environmental signals. This review examines the functional variety of receptors in prokaryotic and eukaryotic systems that exhibit distinct biological responses after interacting with c-di-GMP. Evolutionary relationships and similarities in distance among the c-di-GMP receptors in various bacterial species were evaluated to understand their specificities. Furthermore, residues of receptors involved in c-di-GMP binding are summarized. This review facilitates the understanding of how distinct receptors from different origins bind c-di-GMP equally well, yet fulfill diverse biological roles at the interspecies, intraspecies, and interkingdom levels. Furthermore, it also highlights c-di-GMP receptors as potential therapeutic targets, particularly those found in pathogenic microorganisms. Video Abstract.

During heat stress in Myxococcus xanthus, the CdbS PilZ domain protein, in concert with two PilZ-DnaK chaperones, perturbs chromosome organization and accelerates cell death

C-di-GMP is a bacterial second messenger that regulates diverse processes in response to environmental or cellular cues. The nucleoid-associated protein (NAP) CdbA in Myxococcus xanthus binds c-di-GMP and DNA in a mutually exclusive manner in vitro. CdbA is essential for viability, and CdbA depletion causes defects in chromosome organization, leading to a block in cell division and, ultimately, cell death. Most NAPs are not essential; therefore, to explore the paradoxical cdbA essentiality, we isolated suppressor mutations that restored cell viability without CdbA. Most mutations mapped to cdbS, which encodes a stand-alone c-di-GMP binding PilZ domain protein, and caused loss-of-function of cdbS. Cells lacking CdbA and CdbS or only CdbS were fully viable and had no defects in chromosome organization. CdbA depletion caused post-transcriptional upregulation of CdbS accumulation, and this CdbS over-accumulation was sufficient to disrupt chromosome organization and cause cell death. CdbA depletion also caused increased accumulation of CsdK1 and CsdK2, two unusual PilZ-DnaK chaperones. During CdbA depletion, CsdK1 and CsdK2, in turn, enabled the increased accumulation and toxicity of CdbS, likely by stabilizing CdbS. Moreover, we demonstrate that heat stress, possibly involving an increased cellular c-di-GMP concentration, induced the CdbA/CsdK1/CsdK2/CdbS system, causing a CsdK1- and CsdK2-dependent increase in CdbS accumulation. Thereby this system accelerates heat stress-induced chromosome mis-organization and cell death. Collectively, this work describes a unique system that contributes to regulated cell death in M. xanthus and suggests a link between c-di-GMP signaling and regulated cell death in bacteria.

Subfunctionalization of a monolignol to a phytoalexin glucosyltransferase is accompanied by substrate inhibition

Uridine diphosphate-dependent glycosyltransferases (UGTs) mediate the glycosylation of plant metabolites, thereby altering their physicochemical properties and bioactivities. Plants possess numerous UGT genes, with the encoded enzymes often glycosylating multiple substrates and some exhibiting substrate inhibition kinetics, but the biological function and molecular basis of these phenomena are not fully understood. The promiscuous monolignol/phytoalexin glycosyltransferase NbUGT72AY1 exhibits substrate inhibition (K_i) at 4 μM scopoletin, whereas the highly homologous monolignol StUGT72AY2 is inhibited at 190 μM. We therefore used hydrogen/deuterium exchange mass spectrometry and structure-based mutational analyses of both proteins and introduced NbUGT72AY1 residues into StUGT72AY2 and vice versa to study promiscuity and substrate inhibition of UGTs. A single F87I and chimeric mutant of NbUGT72AY1 showed significantly reduced scopoletin substrate inhibition, whereas its monolignol glycosylation activity was almost unaffected. Reverse mutations in StUGT72AY2 resulted in increased scopoletin glycosylation, leading to enhanced promiscuity, which was accompanied by substrate inhibition. Studies of 3D structures identified open and closed UGT conformers, allowing visualization of the dynamics of conformational changes that occur during catalysis. Previously postulated substrate access tunnels likely serve as drainage channels. The results suggest a two-site model in which the second substrate molecule binds near the catalytic site and blocks product release. Mutational studies showed that minor changes in amino acid sequence can enhance the promiscuity of the enzyme and add new capabilities such as substrate inhibition without affecting existing functions. The proposed subfunctionalization mechanism of expanded promiscuity may play a role in enzyme evolution and highlights the importance of promiscuous enzymes in providing new functions.

Recent advances and perspectives in nucleotide second messenger signaling in bacteria

Nucleotide second messengers act as intracellular 'secondary' signals that represent environmental or cellular cues, i.e. the 'primary' signals. As such, they are linking sensory input with regulatory output in all living cells. The amazing physiological versatility, the mechanistic diversity of second messenger synthesis, degradation, and action as well as the high level of integration of second messenger pathways and networks in prokaryotes has only recently become apparent. In these networks, specific second messengers play conserved general roles. Thus, (p)ppGpp coordinates growth and survival in response to nutrient availability and various stresses, while c-di-GMP is the nucleotide signaling molecule to orchestrate bacterial adhesion and multicellularity. c-di-AMP links osmotic balance and metabolism and that it does so even in Archaea may suggest a very early evolutionary origin of second messenger signaling. Many of the enzymes that make or break second messengers show complex sensory domain architectures, which allow multisignal integration. The multiplicity of c-di-GMP-related enzymes in many species has led to the discovery that bacterial cells are even able to use the same freely diffusible second messenger in local signaling pathways that can act in parallel without cross-talking. On the other hand, signaling pathways operating with different nucleotides can intersect in elaborate signaling networks. Apart from the small number of common signaling nucleotides that bacteria use for controlling their cellular "business," diverse nucleotides were recently found to play very specific roles in phage defense. Furthermore, these systems represent the phylogenetic ancestors of cyclic nucleotide-activated immune signaling in eukaryotes.

Elongation factor P modulates Acinetobacter baumannii physiology and virulence as a cyclic dimeric guanosine monophosphate effector

Cyclic diguanosine monophosphate (c-di-GMP) is widely used by bacteria to control biological functions in response to diverse signals or cues. A previous study showed that potential c-di-GMP metabolic enzymes play a role in the regulation of biofilm formation and motility in Acinetobacter baumannii. However, it was unclear whether and how A. baumannii cells use c-di-GMP signaling to modulate biological functions. Here, we report that c-di-GMP is an important intracellular signal in the modulation of biofilm formation, motility, and virulence in A. baumannii. The intracellular level of c-di-GMP is principally controlled by the diguanylate cyclases (DGCs) A1S_1695, A1S_2506, and A1S_3296 and the phosphodiesterase (PDE) A1S_1254. Intriguingly, we revealed that A1S_2419 (an elongation factor P [EF-P]), is a novel c-di-GMP effector in A. baumannii. Response to a c-di-GMP signal boosted A1S_2419 activity to rescue ribosomes from stalling during synthesis of proteins containing consecutive prolines and thus regulate A. baumannii physiology and pathogenesis. Our study presents a unique and widely conserved effector that controls bacterial physiology and virulence by sensing the second messenger c-di-GMP.

Three PilZ Domain Proteins, PlpA, PixA, and PixB, Have Distinct Functions in Regulation of Motility and Development in Myxococcus xanthus

In bacteria, the nucleotide-based second messenger bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) binds to effectors to generate outputs in response to changes in the environment. In Myxococcus xanthus, c-di-GMP regulates type IV pilus-dependent motility and the starvation-induced developmental program that results in formation of spore-filled fruiting bodies; however, little is known about the effectors that bind c-di-GMP. Here, we systematically inactivated all 24 genes encoding PilZ domain-containing proteins, which are among the most common c-di-GMP effectors. We confirm that the stand-alone PilZ domain protein PlpA is important for regulation of motility independently of the Frz chemosensory system and that Pkn1, which is composed of a Ser/Thr kinase domain and a PilZ domain, is specifically important for development. Moreover, we identify two PilZ domain proteins that have distinct functions in regulating motility and development. PixB, which is composed of two PilZ domains and an acetyltransferase domain, binds c-di-GMP in vitro and regulates type IV pilus-dependent and gliding motility in a Frz-dependent manner as well as development. The acetyltransferase domain is required and sufficient for function during growth, while all three domains and c-di-GMP binding are essential for PixB function during development. PixA is a response regulator composed of a PilZ domain and a receiver domain, binds c-di-GMP in vitro, and regulates motility independently of the Frz system, likely by setting up the polarity of the two motility systems. Our results support a model whereby PlpA, PixA, and PixB act in independent pathways and have distinct functions in regulation of motility. IMPORTANCE c-di-GMP signaling controls bacterial motility in many bacterial species by binding to downstream effector proteins. Here, we identify two PilZ domain-containing proteins in Myxococcus xanthus that bind c-di-GMP. We show that PixB, which contains two PilZ domains and an acetyltransferase domain, acts in a manner that depends on the Frz chemosensory system to regulate motility via the acetyltransferase domain, while the intact protein and c-di-GMP binding are essential for PixB to support development. In contrast, PixA acts in a Frz-independent manner to regulate motility. Taking our results together with previous observations, we conclude that PilZ domain proteins and c-di-GMP act in multiple independent pathways to regulate motility and development in M. xanthus.

CRP-Like Transcriptional Regulator MrpC Curbs c-di-GMP and 3',3'-cGAMP Nucleotide Levels during Development in Myxococcus xanthus

Myxococcus xanthus has a nutrient-regulated biphasic life cycle forming predatory swarms in the presence of nutrients and spore-filled fruiting bodies in the absence of nutrients. The second messenger 3'-5', 3'-5 cyclic di-GMP (c-di-GMP) is essential during both stages of the life cycle; however, different enzymes involved in c-di-GMP synthesis and degradation as well as several c-di-GMP receptors are important during distinct life cycle stages. To address this stage specificity, we determined transcript levels using transcriptome sequencing (RNA-seq) and transcription start sites using Cappable sequencing (Cappable-seq) during growth and development genome wide. All 70 genes encoding c-di-GMP-associated proteins were expressed, with 28 upregulated and 10 downregulated during development. Specifically, the three genes encoding enzymatically active proteins with a stage-specific function were expressed stage specifically. By combining operon mapping with published chromatin immunoprecipitation sequencing (ChIP-seq) data for MrpC (M. Robinson, B. Son, D. Kroos, L. Kroos, BMC Genomics 15:1123, 2014, http://dx.doi.org/10.1186/1471-2164-15-1123), the cAMP receptor protein (CRP)-like master regulator of development, we identified nine developmentally regulated genes as regulated by MrpC. In particular, MrpC directly represses the expression of dmxB, which encodes the diguanylate cyclase DmxB that is essential for development and responsible for the c-di-GMP increase during development. Moreover, MrpC directly activates the transcription of pmxA, which encodes a bifunctional phosphodiesterase that degrades c-di-GMP and 3',3'-cGAMP in vitro and is essential for development. Thereby, MrpC regulates and curbs the cellular pools of c-di-GMP and 3',3'-cGAMP during development. We conclude that temporal regulation of the synthesis of proteins involved in c-di-GMP metabolism contributes to c-di-GMP signaling specificity. MrpC is important for this regulation, thereby being a key regulator of developmental cyclic di-nucleotide metabolism in M. xanthus. IMPORTANCE The second messenger c-di-GMP is important during both stages of the nutrient-regulated biphasic life cycle of Myxococcus xanthus with the formation of predatory swarms in the presence of nutrients and spore-filled fruiting bodies in the absence of nutrients. However, different enzymes involved in c-di-GMP synthesis and degradation are important during distinct life cycle stages. Here, we show that the three genes encoding enzymatically active proteins with a stage-specific function are expressed stage specifically. Moreover, we find that the master transcriptional regulator of development MrpC directly regulates the expression of dmxB, which encodes the diguanylate cyclase DmxB that is essential for development, and of pmxA, which encodes a bifunctional phosphodiesterase that degrades c-di-GMP and 3',3'-cGAMP in vitro and is essential for development. We conclude that temporal regulation of the synthesis of proteins involved in c-di-GMP metabolism contributes to c-di-GMP signaling specificity and that MrpC plays an important role in this regulation.

SMC and the bactofilin/PadC scaffold have distinct yet redundant functions in chromosome segregation and organization in Myxococcus xanthus

In bacteria, ParABS systems and structural maintenance of chromosome (SMC) condensin-like complexes are important for chromosome segregation and organization. The rod-shaped Myxococcus xanthus cells have a unique chromosome arrangement in which a scaffold composed of the BacNOP bactofilins and PadC positions the essential ParB∙parS segregation complexes and the DNA segregation ATPase ParA in the subpolar regions. We identify the Smc and ScpAB subunits of the SMC complex in M. xanthus and demonstrate that SMC is conditionally essential, with Δsmc or ΔscpAB mutants being temperature sensitive. Inactivation of SMC caused defects in chromosome segregation and organization. Lack of the BacNOP/PadC scaffold also caused chromosome segregation defects but this scaffold is not essential for viability. Inactivation of SMC was synthetic lethal with lack of the BacNOP/PadC scaffold. Lack of SMC interfered with formation of the BacNOP/PadC scaffold while lack of this scaffold did not interfere with chromosome association by SMC. Altogether, our data support that three systems function together to enable chromosome segregation in M. xanthus. ParABS constitutes the basic and essential machinery. SMC and the BacNOP/PadC scaffold have different yet redundant roles in chromosome segregation with SMC supporting individualization of daughter chromosomes and BacNOP/PadC making the ParABS system operate more robustly.