Emergence of fractal geometries in the evolution of a metabolic enzyme

Fractals are patterns that are self-similar across multiple length-scales1. Macroscopic fractals are common in nature2-4; however, so far, molecular assembly into fractals is restricted to synthetic systems5-12. Here we report the discovery of a natural protein, citrate synthase from the cyanobacterium Synechococcus elongatus, which self-assembles into Sierpiński triangles. Using cryo-electron microscopy, we reveal how the fractal assembles from a hexameric building block. Although different stimuli modulate the formation of fractal complexes and these complexes can regulate the enzymatic activity of citrate synthase in vitro, the fractal may not serve a physiological function in vivo. We use ancestral sequence reconstruction to retrace how the citrate synthase fractal evolved from non-fractal precursors, and the results suggest it may have emerged as a harmless evolutionary accident. Our findings expand the space of possible protein complexes and demonstrate that intricate and regulatable assemblies can evolve in a single substitution.

Structure of the GDP-bound state of the SRP GTPase FlhF

The GTPase FlhF, a signal recognition particle (SRP)-type enzyme, is pivotal for spatial-numerical control and bacterial flagella assembly across diverse species, including pathogens. This study presents the X-ray structure of FlhF in its GDP-bound state at a resolution of 2.28 Å. The structure exhibits the classical N- and G-domain fold, consistent with related SRP GTPases such as Ffh and FtsY. Comparative analysis with GTP-loaded FlhF elucidates the conformational changes associated with GTP hydrolysis. These topological reconfigurations are similarly evident in Ffh and FtsY, and play a pivotal role in regulating the functions of these hydrolases.

The dual GGDEF/EAL domain enzyme PA0285 is a Pseudomonas species housekeeping phosphodiesterase regulating early attachment and biofilm architecture

Bacterial lifestyles depend on conditions encountered during colonization. The transition between planktonic and biofilm growth is dependent on the intracellular second messenger c-di-GMP. High c-di-GMP levels driven by diguanylate cyclases (DGCs) activity favor biofilm formation, while low levels were maintained by phosphodiesterases (PDE) encourage planktonic lifestyle. The activity of these enzymes can be modulated by stimuli-sensing domains such as Per-ARNT-Sim (PAS). In Pseudomonas aeruginosa, more than 40 PDE/DGC are involved in c-di-GMP homeostasis, including 16 dual proteins possessing both canonical DGC and PDE motifs, that is, GGDEF and EAL, respectively. It was reported that deletion of the EAL/GGDEF dual enzyme PA0285, one of five c-di-GMP-related enzymes conserved across all Pseudomonas species, impacts biofilms. PA0285 is anchored in the membrane and carries two PAS domains. Here, we confirm that its role is conserved in various P. aeruginosa strains and in Pseudomonas putida. Deletion of PA0285 impacts the early stage of colonization, and RNA-seq analysis suggests that expression of cupA fimbrial genes is involved. We demonstrate that the C-terminal portion of PA0285 encompassing the GGDEF and EAL domains binds GTP and c-di-GMP, respectively, but only exhibits PDE activity in vitro. However, both GGDEF and EAL domains are important for PA0285 PDE activity in vivo. Complementation of the PA0285 mutant strain with a copy of the gene encoding the C-terminal GGDEF/EAL portion in trans was not as effective as complementation with the full-length gene. This suggests the N-terminal transmembrane and PAS domains influence the PDE activity in vivo, through modulating the protein conformation.

Proteins that carry dual targeting signals can act as tethers between peroxisomes and partner organelles

Peroxisomes are organelles with crucial functions in oxidative metabolism. To correctly target to peroxisomes, proteins require specialized targeting signals. A mystery in the field is the sorting of proteins that carry a targeting signal for peroxisomes and as well as for other organelles, such as mitochondria or the endoplasmic reticulum (ER). Exploring several of these proteins in fungal model systems, we observed that they can act as tethers bridging organelles together to create contact sites. We show that in Saccharomyces cerevisiae this mode of tethering involves the peroxisome import machinery, the ER-mitochondria encounter structure (ERMES) at mitochondria and the guided entry of tail-anchored proteins (GET) pathway at the ER. Our findings introduce a previously unexplored concept of how dual affinity proteins can regulate organelle attachment and communication.

The virulence regulator VirB from Shigella flexneri uses a CTP-dependent switch mechanism to activate gene expression

The transcriptional antisilencer VirB acts as a master regulator of virulence gene expression in the human pathogen Shigella flexneri. It binds DNA sequences (virS) upstream of VirB-dependent promoters and counteracts their silencing by the nucleoid-organizing protein H-NS. However, its precise mode of action remains unclear. Notably, VirB is not a classical transcription factor but related to ParB-type DNA-partitioning proteins, which have recently been recognized as DNA-sliding clamps using CTP binding and hydrolysis to control their DNA entry gate. Here, we show that VirB binds CTP, embraces DNA in a clamp-like fashion upon its CTP-dependent loading at virS sites and slides laterally on DNA after clamp closure. Mutations that prevent CTP-binding block VirB loading in vitro and abolish the formation of VirB nucleoprotein complexes as well as virulence gene expression in vivo. Thus, VirB represents a CTP-dependent molecular switch that uses a loading-and-sliding mechanism to control transcription during bacterial pathogenesis.

HilE represses the activity of the Salmonella virulence regulator HilD via a mechanism distinct from that of intestinal long-chain fatty acids

The expression of virulence factors essential for the invasion of host cells by Salmonella enterica is tightly controlled by a network of transcription regulators. The AraC/XylS transcription factor HilD is the main integration point of environmental signals into this regulatory network, with many factors affecting HilD activity. Long-chain fatty acids, which are highly abundant throughout the host intestine, directly bind to and repress HilD, acting as environmental cues to coordinate virulence gene expression. The regulatory protein HilE also negatively regulates HilD activity, through a protein-protein interaction. Both of these regulators inhibit HilD dimerization, preventing HilD from binding to target DNA. We investigated the structural basis of these mechanisms of HilD repression. Long-chain fatty acids bind to a conserved pocket in HilD, in a comparable manner to that reported for other AraC/XylS regulators, whereas HilE forms a stable heterodimer with HilD by binding to the HilD dimerization interface. Our results highlight two distinct, mutually exclusive mechanisms by which HilD activity is repressed, which could be exploited for the development of new antivirulence leads.

Structural and functional characterization of MrpR, the master repressor of the Bacillus subtilis prophage SPβ

Prophages control their lifestyle to either be maintained within the host genome or enter the lytic cycle. Bacillus subtilis contains the SPβ prophage whose lysogenic state depends on the MrpR (YopR) protein, a key component of the lysis-lysogeny decision system. Using a historic B. subtilis strain harboring the heat-sensitive SPβ c2 mutant, we demonstrate that the lytic cycle of SPβ c2 can be induced by heat due to a single nucleotide exchange in the mrpR gene, rendering the encoded MrpRG136E protein temperature-sensitive. Structural characterization revealed that MrpR is a DNA-binding protein resembling the overall fold of tyrosine recombinases. MrpR has lost its recombinase function and the G136E exchange impairs its higher-order structure and DNA binding activity. Genome-wide profiling of MrpR binding revealed its association with the previously identified SPbeta repeated element (SPBRE) in the SPβ genome. MrpR functions as a master repressor of SPβ that binds to this conserved element to maintain lysogeny. The heat-inducible excision of the SPβ c2 mutant remains reliant on the serine recombinase SprA. A suppressor mutant analysis identified a previously unknown component of the lysis-lysogeny management system that is crucial for the induction of the lytic cycle of SPβ.

Structural and functional analysis of the cerato-platanin-like protein Cpl1 suggests diverging functions in smut fungi

Plant-pathogenic fungi are causative agents of the majority of plant diseases and can lead to severe crop loss in infected populations. Fungal colonization is achieved by combining different strategies, such as avoiding and counteracting the plant immune system and manipulating the host metabolome. Of major importance are virulence factors secreted by fungi, which fulfil diverse functions to support the infection process. Most of these proteins are highly specialized, with structural and biochemical information often absent. Here, we present the atomic structures of the cerato-platanin-like protein Cpl1 from Ustilago maydis and its homologue Uvi2 from Ustilago hordei. Both proteins adopt a double-Ψβ-barrel architecture reminiscent of cerato-platanin proteins, a class so far not described in smut fungi. Our structure-function analysis shows that Cpl1 binds to soluble chitin fragments via two extended grooves at the dimer interface of the two monomer molecules. This carbohydrate-binding mode has not been observed previously and expands the repertoire of chitin-binding proteins. Cpl1 localizes to the cell wall of U. maydis and might synergize with cell wall-degrading and decorating proteins during maize infection. The architecture of Cpl1 harbouring four surface-exposed loop regions supports the idea that it might play a role in the spatial coordination of these proteins. While deletion of cpl1 has only mild effects on the virulence of U. maydis, a recent study showed that deletion of uvi2 strongly impairs U. hordei virulence. Our structural comparison between Cpl1 and Uvi2 reveals sequence variations in the loop regions that might explain a diverging function.

Peptide-mediated inhibition of the transcriptional regulator Elongin BC induces apoptosis in cancer cells

Inhibition of protein-protein interactions (PPIs) via designed peptides is an effective strategy to perturb their biological functions. The Elongin BC heterodimer (ELOB/C) binds to a BC-box motif and is essential for cancer cell growth. Here, we report a peptide that mimics the high-affinity BC-box of the PRC2-associated protein EPOP. This peptide tightly binds to the ELOB/C dimer (k_D = 0.46 ± 0.02 nM) and blocks the association of ELOB/C with its interaction partners, both in vitro and in the cellular environment. Cancer cells treated with our peptide inhibitor showed decreased cell viability, increased apoptosis, and perturbed gene expression. Therefore, our work proposes that blocking the BC-box-binding pocket of ELOB/C is a feasible strategy to impair its function and inhibit cancer cell growth. Our peptide inhibitor promises novel mechanistic insights into the biological function of the ELOB/C dimer and offers a starting point for therapeutics linked to ELOB/C dysfunction.

(p)ppGpp and moonlighting RNases influence the first step of lipopolysaccharide biosynthesis in Escherichia coli

The outer membrane (OM) protects Gram-negative bacteria from harsh environmental conditions and provides intrinsic resistance to many antimicrobial compounds. The asymmetric OM is characterized by phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet. Previous reports suggested an involvement of the signaling nucleotide ppGpp in cell envelope homeostasis in Escherichia coli. Here, we investigated the effect of ppGpp on OM biosynthesis. We found that ppGpp inhibits the activity of LpxA, the first enzyme of LPS biosynthesis, in a fluorometric in vitro assay. Moreover, overproduction of LpxA resulted in elongated cells and shedding of outer membrane vesicles (OMVs) with altered LPS content. These effects were markedly stronger in a ppGpp-deficient background. We further show that RnhB, an RNase H isoenzyme, binds ppGpp, interacts with LpxA, and modulates its activity. Overall, our study uncovered new regulatory players in the early steps of LPS biosynthesis, an essential process with many implications in the physiology and susceptibility to antibiotics of Gram-negative commensals and pathogens.

The mysterious diadenosine tetraphosphate (AP4A)

Dinucleoside polyphosphates, a class of nucleotides found amongst all the Trees of Life, have been gathering a lot of attention in the past decades due to their putative role as cellular alarmones. In particular, diadenosine tetraphosphate (AP4A) has been widely studied in bacteria facing various environmental challenges and has been proposed to be important for ensuring cellular survivability through harsh conditions. Here, we discuss the current understanding of AP4A synthesis and degradation, protein targets, their molecular structure where possible, and insights into the molecular mechanisms of AP4A action and its physiological consequences. Lastly, we will briefly touch on what is known with regards to AP4A beyond the bacterial kingdom, given its increasing appearance in the eukaryotic world. Altogether, the notion that AP4A is a conserved second messenger in organisms ranging from bacteria to humans and is able to signal and modulate cellular stress regulation seems promising.

Fortuitously compatible protein surfaces primed allosteric control in cyanobacterial photoprotection

Highly specific interactions between proteins are a fundamental prerequisite for life, but how they evolve remains an unsolved problem. In particular, interactions between initially unrelated proteins require that they evolve matching surfaces. It is unclear whether such surface compatibilities can only be built by selection in small incremental steps, or whether they can also emerge fortuitously. Here, we used molecular phylogenetics, ancestral sequence reconstruction and biophysical characterization of resurrected proteins to retrace the evolution of an allosteric interaction between two proteins that act in the cyanobacterial photoprotection system. We show that this interaction between the orange carotenoid protein (OCP) and its unrelated regulator, the fluorescence recovery protein (FRP), evolved when a precursor of FRP was horizontally acquired by cyanobacteria. FRP's precursors could already interact with and regulate OCP even before these proteins first encountered each other in an ancestral cyanobacterium. The OCP-FRP interaction exploits an ancient dimer interface in OCP, which also predates the recruitment of FRP into the photoprotection system. Together, our work shows how evolution can fashion complex regulatory systems easily out of pre-existing components.

Cryo-EM structure of human eIF5A-DHS complex reveals the molecular basis of hypusination-associated neurodegenerative disorders

Hypusination is a unique post-translational modification of the eukaryotic translation factor 5A (eIF5A) that is essential for overcoming ribosome stalling at polyproline sequence stretches. The initial step of hypusination, the formation of deoxyhypusine, is catalyzed by deoxyhypusine synthase (DHS), however, the molecular details of the DHS-mediated reaction remained elusive. Recently, patient-derived variants of DHS and eIF5A have been linked to rare neurodevelopmental disorders. Here, we present the cryo-EM structure of the human eIF5A-DHS complex at 2.8 Å resolution and a crystal structure of DHS trapped in the key reaction transition state. Furthermore, we show that disease-associated DHS variants influence the complex formation and hypusination efficiency. Hence, our work dissects the molecular details of the deoxyhypusine synthesis reaction and reveals how clinically-relevant mutations affect this crucial cellular process.

At the Crossroad of Nucleotide Dynamics and Protein Synthesis in Bacteria

Nucleotides are at the heart of the most essential biological processes in the cell, be it as key protagonists in the dogma of molecular biology or by regulating multiple metabolic pathways. The dynamic nature of nucleotides, the cross talk between them, and their constant feedback to and from the cell's metabolic state position them as a hallmark of adaption toward environmental and growth challenges. It has become increasingly clear how the activity of RNA polymerase, the synthesis and maintenance of tRNAs, mRNA translation at all stages, and the biogenesis and assembly of ribosomes are fine-tuned by the pools of intracellular nucleotides. With all aspects composing protein synthesis involved, the ribosome emerges as the molecular hub in which many of these nucleotides encounter each other and regulate the state of the cell. In this review, we aim to highlight intracellular nucleotides in bacteria as dynamic characters permanently cross talking with each other and ultimately regulating protein synthesis at various stages in which the ribosome is mainly the principal character.

MerA functions as a hypothiocyanous acid reductase and defense mechanism in Staphylococcus aureus

The major pathogen Staphylococcus aureus has to cope with host-derived oxidative stress to cause infections in humans. Here, we report that S. aureus tolerates high concentrations of hypothiocyanous acid (HOSCN), a key antimicrobial oxidant produced in the respiratory tract. We discovered that the flavoprotein disulfide reductase (FDR) MerA protects S. aureus from this oxidant by functioning as a HOSCN reductase, with its deletion sensitizing bacteria to HOSCN. Crystal structures of homodimeric MerA (2.4 Å) with a Cys₄₃ -Cys₄₈ intramolecular disulfide, and reduced MerACys₄₃ S (1.6 Å) showed the FAD cofactor close to the active site, supporting that MerA functions as a group I FDR. MerA is controlled by the redox-sensitive repressor HypR, which we show to be oxidized to intermolecular disulfides under HOSCN stress, resulting in its inactivation and derepression of merA transcription to promote HOSCN tolerance. Our study highlights the HOSCN tolerance of S. aureus and characterizes the structure and function of MerA as a major HOSCN defense mechanism. Crippling the capacity to respond to HOSCN may be a novel strategy for treating S. aureus infections.

Evolutionary reconstruction, nomenclature and functional meta-analysis of the Kiwellin protein family

Crop diseases caused by pathogens critically affect global food security and plant ecology. Pathogens are well adapted to their host plants and have developed sophisticated mechanisms allowing successful colonization. Plants in turn have taken measures to counteract pathogen attacks resulting in an evolutionary arms race. Recent studies provided mechanistic insights into how two plant Kiwellin proteins from Zea mays mitigate the activity of the chorismate mutase Cmu1, a virulence factor secreted by the fungal pathogen Ustilago maydis during maize infection. Formerly identified as human allergens in kiwifruit, the biological function of Kiwellins is apparently linked to plant defense. We combined the analysis of proteome data with structural predictions to obtain a holistic overview of the Kiwellin protein family, that is subdivided into proteins with and without a N-terminal kissper domain. We found that Kiwellins are evolutionarily conserved in various plant species. At median five Kiwellin paralogs are encoded in each plant genome. Structural predictions revealed that Barwin-like proteins and Kiwellins cannot be discriminated purely at the sequence level. Our data shows that Kiwellins emerged in land plants (embryophyta) and are not present in fungi as suggested earlier. They evolved via three major duplication events that lead to clearly distinguishable subfamilies. We introduce a systematic Kiwellin nomenclature based on a detailed evolutionary reconstruction of this protein family. A meta-analysis of publicly available transcriptome data demonstrated that Kiwellins can be differentially regulated upon the interaction of plants with pathogens but also with symbionts. Furthermore, significant differences in Kiwellin expression levels dependent on tissues and cultivars were observed. In summary, our study sheds light on the evolution and regulation of a large protein family and provides a framework for a more detailed understanding of the molecular functions of Kiwellins.

Genome Mining Enabled by Biosynthetic Characterization Uncovers a Class of Benzoxazolinate-Containing Natural Products in Diverse Bacteria

Benzoxazolinate is a rare bis-heterocyclic moiety that interacts with proteins and DNA and confers extraordinary bioactivities on natural products, such as C-1027. However, the biosynthetic gene responsible for the key cyclization step of benzoxazolinate remains unclear. Herein, we show a putative acyl AMP-ligase responsible for the last cyclization step. We used the enzyme as a probe for genome mining and discovered that the orphan benzobactin gene cluster in entomopathogenic bacteria prevails across Proteobacteria and Firmicutes. It turns out that Pseudomonas chlororaphis produces various benzobactins, whose biosynthesis is highlighted by a synergistic effect of two unclustered genes encoding enzymes on boosting benzobactin production; the formation of non-proteinogenic 2-hydroxymethylserine by a serine hydroxymethyltransferase; and the types I and II NRPS architecture for structural diversity. Our findings reveal the biosynthetic potential of a widespread benzobactin gene cluster.

Screening for eukaryotic motifs in Legionella pneumophila reveals Smh1 as bacterial deacetylase of host histones

Legionella pneumophila (L.p.) is a bacterial pathogen which is a common causative agent of pneumonia. In humans, it infects alveolar macrophages and transfers hundreds of virulence factors that interfere with cellular signalling pathways and the transcriptomic landscape to sustain its own replication. By this interaction, it has acquired eukaryote-like protein motifs by gene transfer events that partake in the pathogenicity of Legionella. In a computational screening approach for eukaryotic motifs in the transcriptome of Legionella, we identified the L.p. strain Corby protein ABQ55614 as putative histone-deacetylase and named it "suppressing modifier of histones 1" (Smh1). During infection, Smh1 is translocated from the Legionella vacuole into the host cytosol. When expressed in human macrophage THP-1 cells, Smh1 was localized predominantly in the nucleus, leading to broad histone H3 and H4 deacetylation, blunted expression of a large number of genes (e.g. IL-1β and IL-8), and fostered intracellular bacterial replication. L.p. with a Smh1 knockdown grew normally in media but showed a slight growth defect inside the host cell. Furthermore, Smh1 showed a very potent histone deacetylation activity in vitro, e.g. at H3K14, that could be inhibited by targeted mutation of the putative catalytic center inferred by analogy with eukaryotic HDAC8, and with the deacetylase inhibitor trichostatin A. In summary, Smh1 displays functional homology with class I/II type HDACs. We identified Smh1 as a new Legionella virulence factor with a eukaryote-like histone-deacetylase activity that moderates host gene expression and might pave the way for further histone modifications.IMPORTANCELegionella pneumophila (L.p.) is a prominent bacterial pathogen, which is a common causative agent of pneumonia. In order to survive inside the host cell, the human macrophage, it profoundly interacts with host cell processes to advance its own replication. In this study, we identify a bacterial factor, Smh1, with yet unknown function as a host histone deacetylase. The activity of this factor in the host cell leads to attenuated gene expression and increased intracellular bacterial replication.

Single-molecule dynamics suggest that ribosomes assemble at sites of translation in Bacillus subtilis

Eukaryotic cells transcribe ribosomal RNA and largely assemble ribosomes in a structure called the nucleolus, where chromosomal regions containing rRNA operons are clustered. In bacteria, many rRNA operons cluster close to the origin regions that are positioned on the outer borders of nucleoids, close to polar areas, where translating 70S ribosomes are located. Because outer regions of the nucleoids contain the highest accumulation of RNA polymerase, it has been hypothesized that bacteria contain "nucleolus-like" structures. However, ribosome subunits freely diffuse through the entire cells, and could thus be assembled and matured throughout the non-compartmentalized cell. By tracking single molecules of two GTPases that play an essential role in ribosomal folding and processing in Bacillus subtilis, we show that this process takes place at sites of translation, i.e., predominantly at the cell poles. Induction of the stringent response led to a change in the population of GTPases assumed to be active in maturation, but did not abolish nucleoid occlusion of ribosomes or of GTPases. Our findings strongly support the idea of the conceptualization of nucleolus-like structures in bacteria, i.e., rRNA synthesis, ribosomal protein synthesis and subunit assembly occurring in close proximity at the cell poles, facilitating the efficiency of ribosome maturation even under conditions of transient nutrient deprivation.

The iron-sulfur cluster assembly (ISC) protein Iba57 executes a tetrahydrofolate-independent function in mitochondrial [4Fe-4S] protein maturation

Mitochondria harbor the bacteria-inherited iron-sulfur cluster assembly (ISC) machinery to generate [2Fe-2S] and [4Fe-4S] proteins. In yeast, assembly of [4Fe-4S] proteins specifically involves the ISC proteins Isa1, Isa2, Iba57, Bol3, and Nfu1. Functional defects in their human equivalents cause the multiple mitochondrial dysfunction syndromes (MMDS), severe disorders with a broad clinical spectrum. The bacterial Iba57 ancestor YgfZ was described to require tetrahydrofolate (THF) for its function in the maturation of selected [4Fe-4S] proteins. Both YgfZ and Iba57 are structurally related to an enzyme family catalyzing THF-dependent one-carbon transfer reactions including GcvT of the glycine cleavage system. On this basis, a universally conserved folate requirement in ISC-dependent [4Fe-4S] protein biogenesis was proposed. To test this idea for mitochondrial Iba57, we performed genetic and biochemical studies in S. cerevisiae, and we solved the crystal structure of Iba57 from the thermophilic fungus Chaetomium thermophilum. We provide three lines of evidence for the THF independence of the Iba57-catalyzed [4Fe-4S] protein assembly pathway. First, yeast mutants lacking folate show no defect in mitochondrial [4Fe-4S] protein maturation. Second, the 3D structure of Iba57 lacks many of the side chain contacts to THF as defined in GcvT, and the THF binding pocket is constricted. Third, mutations in conserved Iba57 residues that are essential for THF-dependent catalysis in GcvT do not impair Iba57 function in vivo, in contrast to an exchange of the invariant, surface-exposed cysteine residue. We conclude that mitochondrial Iba57, despite structural similarities to both YgfZ and THF-binding proteins, does not utilize folate for its function.

Diadenosine tetraphosphate regulates biosynthesis of GTP in Bacillus subtilis

Diadenosine tetraphosphate (Ap4A) is a putative second messenger molecule that is conserved from bacteria to humans. Nevertheless, its physiological role and the underlying molecular mechanisms are poorly characterized. We investigated the molecular mechanism by which Ap4A regulates inosine-5'-monophosphate dehydrogenase (IMPDH, a key branching point enzyme for the biosynthesis of adenosine or guanosine nucleotides) in Bacillus subtilis. We solved the crystal structure of BsIMPDH bound to Ap4A at a resolution of 2.45 Å to show that Ap4A binds to the interface between two IMPDH subunits, acting as the glue that switches active IMPDH tetramers into less active octamers. Guided by these insights, we engineered mutant strains of B. subtilis that bypass Ap4A-dependent IMPDH regulation without perturbing intracellular Ap4A pools themselves. We used metabolomics, which suggests that these mutants have a dysregulated purine, and in particular GTP, metabolome and phenotypic analysis, which shows increased sensitivity of B. subtilis IMPDH mutant strains to heat compared with wild-type strains. Our study identifies a central role for IMPDH in remodelling metabolism and heat resistance, and provides evidence that Ap4A can function as an alarmone.

GGDEF domain as spatial on-switch for a phosphodiesterase by interaction with landmark protein HubP

In bacteria, the monopolar localization of enzymes and protein complexes can result in a bimodal distribution of enzyme activity between the dividing cells and heterogeneity of cellular behaviors. In Shewanella putrefaciens, the multidomain hybrid diguanylate cyclase/phosphodiesterase PdeB, which degrades the secondary messenger c-di-GMP, is located at the flagellated cell pole. Here, we show that direct interaction between the inactive diguanylate cyclase (GGDEF) domain of PdeB and the FimV domain of the polar landmark protein HubP is crucial for full function of PdeB as a phosphodiesterase. Thus, the GGDEF domain serves as a spatially controlled on-switch that effectively restricts PdeBs activity to the flagellated cell pole. PdeB regulates abundance and activity of at least two crucial surface-interaction factors, the BpfA surface-adhesion protein and the MSHA type IV pilus. The heterogeneity in c-di-GMP concentrations, generated by differences in abundance and timing of polar appearance of PdeB, orchestrates the population behavior with respect to cell-surface interaction and environmental spreading.

In Vitro Synergy of Isavuconazole Combined With Colistin Against Common Candida Species

Interactions of isavuconazole and colistin were evaluated against 57 common Candida strains belonging to the species Candida albicans (n = 10), Candida glabrata (n = 10), Candida kefyr (n = 8), Candida krusei (n = 10), Candida parapsilosis (n = 9), and Candida tropicalis (n = 10) by a broth microdilution checkerboard technique based on the European Committee on Antimicrobial Susceptibility Testing (EUCAST) reference methodology for antifungal susceptibility testing. Results were analyzed with the fractional inhibitory concentration index and by the response surface analysis. Interpretation by the fractional inhibitory concentration index showed synergy for 50%, 80%, 90%, and 90% of the C. kefyr, C. krusei, C. glabrata, and C. tropicalis strains, respectively. Combination of isavuconazole with colistin against C. albicans and C. parapsilosis exhibited only indifference for 100% and 90% of the strains, respectively. The results were confirmed by response surface analysis for all species except for C. glabrata, for which an indifferent interaction was found for the majority of strains. Antagonistic interaction was never seen regardless of the interpretation model was used.

The many faces of the unusual biofilm activator RemA

Biofilms can be viewed as tissue-like structures in which microorganisms are organized in a spatial and functional sophisticated manner. Biofilm formation requires the orchestration of a highly integrated network of regulatory proteins to establish cell differentiation and production of a complex extracellular matrix. Here, we discuss the role of the essential Bacillus subtilis biofilm activator RemA. Despite intense research on biofilms, RemA is a largely underappreciated regulatory protein. RemA forms donut-shaped octamers with the potential to assemble into dimeric superstructures. The presumed DNA-binding mode suggests that RemA organizes its target DNA into nucleosome-like structures, which are the basis for its role as transcriptional activator. We discuss how RemA affects gene expression in the context of biofilm formation, and its regulatory interplay with established components of the biofilm regulatory network, such as SinR, SinI, SlrR, and SlrA. We emphasize the additional role of RemA played in nitrogen metabolism and osmotic-stress adjustment.

Bistable Photoswitch Allows in Vivo Control of Hematopoiesis

Optical control has enabled functional modulation in cell culture with unparalleled spatiotemporal resolution. However, current tools for in vivo manipulation are scarce. Here, we design and implement a genuine on-off optochemical probe capable of achieving hematopoietic control in zebrafish. Our photopharmacological approach first developed conformationally strained visible light photoswitches (CS-VIPs) as inhibitors of the histone methyltransferase MLL1 (KMT2A). In blood homeostasis MLL1 plays a crucial yet controversial role. CS-VIP 8 optimally fulfils the requirements of a true bistable functional system in vivo under visible-light irradiation, and with unprecedented stability. These properties are exemplified via hematopoiesis photoinhibition with a single isomer in zebrafish. The present interdisciplinary study uncovers the mechanism of action of CS-VIPs. Upon WDR5 binding, CS-VIP 8 causes MLL1 release with concomitant allosteric rearrangements in the WDR5/RbBP5 interface. Since our tool provides on-demand reversible control without genetic intervention or continuous irradiation, it will foster hematopathology and epigenetic investigations. Furthermore, our workflow will enable exquisite photocontrol over other targets inhibited by macrocycles.

The CTPase activity of ParB determines the size and dynamics of prokaryotic DNA partition complexes

ParB-like CTPases mediate the segregation of bacterial chromosomes and low-copy number plasmids. They act as DNA-sliding clamps that are loaded at parS motifs in the centromere of target DNA molecules and spread laterally to form large nucleoprotein complexes serving as docking points for the DNA segregation machinery. Here, we solve crystal structures of ParB in the pre- and post-hydrolysis state and illuminate the catalytic mechanism of nucleotide hydrolysis. Moreover, we identify conformational changes that underlie the CTP- and parS-dependent closure of ParB clamps. The study of CTPase-deficient ParB variants reveals that CTP hydrolysis serves to limit the sliding time of ParB clamps and thus drives the establishment of a well-defined ParB diffusion gradient across the centromere whose dynamics are critical for DNA segregation. These findings clarify the role of the ParB CTPase cycle in partition complex assembly and function and thus advance our understanding of this prototypic CTP-dependent molecular switch.

Two P or Not Two P: Understanding Regulation by the Bacterial Second Messengers (p)ppGpp

Under stressful growth conditions and nutrient starvation, bacteria adapt by synthesizing signaling molecules that profoundly reprogram cellular physiology. At the onset of this process, called the stringent response, members of the RelA/SpoT homolog (RSH) protein superfamily are activated by specific stress stimuli to produce several hyperphosphorylated forms of guanine nucleotides, commonly referred to as (p)ppGpp. Some bifunctional RSH enzymes also harbor domains that allow for degradation of (p)ppGpp by hydrolysis. (p)ppGpp synthesis or hydrolysis may further be executed by single-domain alarmone synthetases or hydrolases, respectively. The downstream effects of (p)ppGpp rely mainly on direct interaction with specific intracellular effectors, which are widely used throughout most cellular processes. The growing number of identified (p)ppGpp targets allows us to deduce both common features of and differences between gram-negative and gram-positive bacteria. In this review, we give an overview of (p)ppGpp metabolism with a focus on the functional and structural aspects of the enzymes involved and discuss recent findings on alarmone-regulated cellular effectors. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Structure and mechanistic features of the prokaryotic minimal RNase P

Endonucleolytic removal of 5'-leader sequences from tRNA precursor transcripts (pre-tRNAs) by ribonuclease P (RNase P) is essential for protein synthesis. Beyond RNA-based RNase P enzymes, protein-only versions of the enzyme exert this function in various eukarya (there termed PRORPs) and in some bacteria (Aquifex aeolicus and close relatives); both enzyme types belong to distinct subgroups of the PIN domain metallonuclease superfamily. Homologs of Aquifex RNase P (HARPs) are also expressed in some other bacteria and many archaea, where they coexist with RNA-based RNase P and do not represent the main RNase P activity. Here, we solved the structure of the bacterial HARP from Halorhodospira halophila by cryo-electron microscopy, revealing a novel screw-like dodecameric assembly. Biochemical experiments demonstrate that oligomerization is required for RNase P activity of HARPs. We propose that the tRNA substrate binds to an extended spike-helix (SH) domain that protrudes from the screw-like assembly to position the 5'-end in close proximity to the active site of the neighboring dimer. The structure suggests that eukaryotic PRORPs and prokaryotic HARPs recognize the same structural elements of pre-tRNAs (tRNA elbow region and cleavage site). Our analysis thus delivers the structural and mechanistic basis for pre-tRNA processing by the prokaryotic HARP system.

The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

Ectoine and its derivative 5-hydroxyectoine are compatible solutes and chemical chaperones widely synthesized by Bacteria and some Archaea as cytoprotectants during osmotic stress and high- or low-growth temperature extremes. The function-preserving attributes of ectoines led to numerous biotechnological and biomedical applications and fostered the development of an industrial scale production process. Synthesis of ectoines requires the expenditure of considerable energetic and biosynthetic resources. Hence, microorganisms have developed ways to exploit ectoines as nutrients when they are no longer needed as stress protectants. Here, we summarize our current knowledge on the phylogenomic distribution of ectoine producing and consuming microorganisms. We emphasize the structural enzymology of the pathways underlying ectoine biosynthesis and consumption, an understanding that has been achieved only recently. The synthesis and degradation pathways critically differ in the isomeric form of the key metabolite N-acetyldiaminobutyric acid (ADABA). γ-ADABA serves as preferred substrate for the ectoine synthase, while the α-ADABA isomer is produced by the ectoine hydrolase as an intermediate in catabolism. It can serve as internal inducer for the genetic control of ectoine catabolic genes via the GabR/MocR-type regulator EnuR. Our review highlights the importance of structural enzymology to inspire the mechanistic understanding of metabolic networks at the biological scale.

Physiology of guanosine-based second messenger signaling in Bacillus subtilis

The guanosine-based second messengers (p)ppGpp and c-di-GMP are key players of the physiological regulation of the Gram-positive model organism Bacillus subtilis. Their regulatory spectrum ranges from key metabolic processes over motility to biofilm formation. Here we review our mechanistic knowledge on their synthesis and degradation in response to environmental and stress signals as well as what is known on their cellular effectors and targets. Moreover, we discuss open questions and our gaps in knowledge on these two important second messengers.

Metabolism of non-growing bacteria

A main function of bacterial metabolism is to supply biomass building blocks and energy for growth. This seems to imply that metabolism is idle in non-growing bacteria. But how relevant is metabolism for the physiology of non-growing bacteria and how active is their metabolism? Here, we reviewed literature describing metabolism of non-growing bacteria in their natural environment, as well as in biotechnological and medical applications. We found that metabolism does play an important role during dormancy and that especially the demand for ATP determines metabolic activity of non-growing bacteria.

The Stand-Alone PilZ-Domain Protein MotL Specifically Regulates the Activity of the Secondary Lateral Flagellar System in Shewanella putrefaciens

A number of bacterial species control the function of the flagellar motor in response to the levels of the secondary messenger c-di-GMP, which is often mediated by c-di-GMP-binding proteins that act as molecular brakes or clutches to slow the motor rotation. The gammaproteobacterium Shewanella putrefaciens possesses two distinct flagellar systems, the primary single polar flagellum and a secondary system with one to five lateral flagellar filaments. Here, we identified a protein, MotL, which specifically regulates the activity of the lateral, but not the polar, flagellar motors in response to the c-di-GMP levels. MotL only consists of a single PilZ domain binding c-di-GMP, which is crucial for its function. Deletion and overproduction analyses revealed that MotL slows down the lateral flagella at elevated levels of c-di-GMP, and may speed up the lateral flagellar-mediated movement at low c-di-GMP concentrations. In vitro interaction studies hint at an interaction of MotL with the C-ring of the lateral flagellar motors. This study shows a differential c-di-GMP-dependent regulation of the two flagellar systems in a single species, and implicates that PilZ domain-only proteins can also act as molecular regulators to control the flagella-mediated motility in bacteria.

Structural Basis for Regulation of the Opposing (p)ppGpp Synthetase and Hydrolase within the Stringent Response Orchestrator Rel

The stringent response enables metabolic adaptation of bacteria under stress conditions and is governed by RelA/SpoT Homolog (RSH)-type enzymes. Long RSH-type enzymes encompass an N-terminal domain (NTD) harboring the second messenger nucleotide (p)ppGpp hydrolase and synthetase activity and a stress-perceiving and regulatory C-terminal domain (CTD). CTD-mediated binding of Rel to stalled ribosomes boosts (p)ppGpp synthesis. However, how the opposing activities of the NTD are controlled in the absence of stress was poorly understood. Here, we demonstrate on the RSH-type protein Rel that the critical regulative elements reside within the TGS (ThrRS, GTPase, and SpoT) subdomain of the CTD, which associates to and represses the synthetase to concomitantly allow for activation of the hydrolase. Furthermore, we show that Rel forms homodimers, which appear to control the interaction with deacylated-tRNA, but not the enzymatic activity of Rel. Collectively, our study provides a detailed molecular view into the mechanism of stringent response repression in the absence of stress.

Dynamics of Bacterial Signal Recognition Particle at a Single Molecule Level

We have studied the localization and dynamics of bacterial Ffh, part of the SRP complex, its receptor FtsY, and of ribosomes in the Gamma-proteobacterium Shewanella putrefaciens. Using structured illumination microscopy, we show that ribosomes show a pronounced accumulation at the cell poles, whereas SRP and FtsY are distributed at distinct sites along the cell membrane, but they are not accumulated at the poles. Single molecule dynamics can be explained by assuming that all three proteins/complexes move as three distinguishable mobility fractions: a low mobility/static fraction may be engaged in translation, medium-fast diffusing fractions may be transition states, and high mobility populations likely represent freely diffusing molecules/complexes. Diffusion constants suggest that SRP and FtsY move together with slow-mobile ribosomes. Inhibition of transcription leads to loss of static molecules and reduction of medium-mobile fractions, in favor of freely diffusing subunits, while inhibition of translation appears to stall the medium mobile fractions. Depletion of FtsY leads to aggregation of Ffh, but not to loss of the medium mobile fraction, indicating that Ffh/SRP can bind to ribosomes independently from FtsY. Heat maps visualizing the three distinct diffusive populations show that while static molecules are mostly clustered at the cell membrane, diffusive molecules are localized throughout the cytosol. The medium fast populations show an intermediate pattern of preferential localization, suggesting that SRP/FtsY/ribosome transition states may form within the cytosol to finally find a translocon.

Dual role of a (p)ppGpp- and (p)ppApp-degrading enzyme in biofilm formation and interbacterial antagonism

The guanosine nucleotide-based second messengers ppGpp and pppGpp (collectively: (p)ppGpp) enable adaptation of microorganisms to environmental changes and stress conditions. In contrast, the closely related adenosine nucleotides (p)ppApp are involved in type VI secretion system (T6SS)-mediated killing during bacterial competition. Long RelA-SpoT Homolog (RSH) enzymes regulate synthesis and degradation of (p)ppGpp (and potentially also (p)ppApp) through their synthetase and hydrolase domains, respectively. Small alarmone hydrolases (SAH) that consist of only a hydrolase domain are found in a variety of bacterial species, including the opportunistic human pathogen Pseudomonas aeruginosa. Here, we present the structure and mechanism of P. aeruginosa SAH showing that the enzyme promiscuously hydrolyses (p)ppGpp and (p)ppApp in a strictly manganese-dependent manner. While being dispensable for P. aeruginosa growth or swimming, swarming, and twitching motilities, its enzymatic activity is required for biofilm formation. Moreover, (p)ppApp-degradation by SAH provides protection against the T6SS (p)ppApp synthetase effector Tas1, suggesting that SAH enzymes can also serve as defense proteins during interbacterial competition.

A Proline-Rich Element in the Type III Secretion Protein FlhB Contributes to Flagellar Biogenesis in the Beta- and Gamma-Proteobacteria

Flagella are bacterial organelles of locomotion. Their biogenesis is highly coordinated in time and space and relies on a specialized flagellar type III secretion system (fT3SS) required for the assembly of the extracellular hook, rod, and filament parts of this complex motor device. The fT3SS protein FlhB switches secretion substrate specificity once the growing hook reaches its determined length. Here we present the crystal structure of the cytoplasmic domain of the transmembrane protein FlhB. The structure visualizes a so-far unseen proline-rich region (PRR) at the very C-terminus of the protein. Strains lacking the PRR show a decrease in flagellation as determined by hook- and filament staining, indicating a role of the PRR during assembly of the hook and filament structures. Phylogenetic analysis shows that the PRR is a primary feature of FlhB proteins of flagellated beta- and gamma-proteobacteria. Taken together, our study adds another layer of complexity and organismic diversity to the process of flagella biogenesis.

Architecture of the active post-translational Sec translocon

In eukaryotes, most secretory and membrane proteins are targeted by an N‐terminal signal sequence to the endoplasmic reticulum, where the trimeric Sec61 complex serves as protein‐conducting channel (PCC). In the post‐translational mode, fully synthesized proteins are recognized by a specialized channel additionally containing the Sec62, Sec63, Sec71, and Sec72 subunits. Recent structures of this Sec complex in the idle state revealed the overall architecture in a pre‐opened state. Here, we present a cryo‐EM structure of the yeast Sec complex bound to a substrate, and a crystal structure of the Sec62 cytosolic domain. The signal sequence is inserted into the lateral gate of Sec61α similar to previous structures, yet, with the gate adopting an even more open conformation. The signal sequence is flanked by two Sec62 transmembrane helices, the cytoplasmic N‐terminal domain of Sec62 is more rigidly positioned, and the plug domain is relocated. We crystallized the Sec62 domain and mapped its interaction with the C‐terminus of Sec63. Together, we obtained a near‐complete and integrated model of the active Sec complex.

Molecular architecture of the DNA-binding sites of the P-loop ATPases MipZ and ParA from Caulobacter crescentus

The spatiotemporal regulation of chromosome segregation and cell division in Caulobacter crescentus is mediated by two different P-loop ATPases, ParA and MipZ. Both of these proteins form dynamic concentration gradients that control the positioning of regulatory targets within the cell. Their proper localization depends on their nucleotide-dependent cycling between a monomeric and a dimeric state and on the ability of the dimeric species to associate with the nucleoid. In this study, we use a combination of genetic screening, biochemical analysis and hydrogen/deuterium exchange mass spectrometry to comprehensively map the residues mediating the interactions of MipZ and ParA with DNA. We show that MipZ has non-specific DNA-binding activity that relies on an array of positively charged and hydrophobic residues lining both sides of the dimer interface. Extending our analysis to ParA, we find that the MipZ and ParA DNA-binding sites differ markedly in composition, although their relative positions on the dimer surface and their mode of DNA binding are conserved. In line with previous experimental work, bioinformatic analysis suggests that the same principles may apply to other members of the P-loop ATPase family. P-loop ATPases thus share common mechanistic features, although their functions have diverged considerably during the course of evolution.

(p)ppGpp: Magic Modulators of Bacterial Physiology and Metabolism

When bacteria experience growth-limiting environmental conditions, the synthesis of the hyperphosphorylated guanosine derivatives (p)ppGpp is induced by enzymes of the RelA/SpoT homology (RSH)-type protein family. High levels of (p)ppGpp induce a process called "stringent response", a major cellular reprogramming during which ribosomal RNA (rRNA) and transfer RNA (tRNA) synthesis is downregulated, stress-related genes upregulated, messenger RNA (mRNA) stability and translation altered, and allocation of scarce resources optimized. The (p)ppGpp-mediated stringent response is thus often regarded as an all-or-nothing paradigm induced by stress. Over the past decades, several binding partners of (p)ppGpp have been uncovered displaying dissociation constants from below one micromolar to more than one millimolar and thus coincide with the accepted intracellular concentrations of (p)ppGpp under non-stringent (basal levels) and stringent conditions. This suggests that the ability of (p)ppGpp to modulate target proteins or processes would be better characterized as an unceasing continuum over a concentration range instead of being an abrupt switch of biochemical processes under specific conditions. We analyzed the reported binding affinities of (p)ppGpp targets and depicted a scheme for prioritization of modulation by (p)ppGpp. In this ranking, many enzymes of e.g., nucleotide metabolism are among the first targets to be affected by rising (p)ppGpp while more fundamental processes such as DNA replication are among the last. This preference should be part of (p)ppGpp's "magic" in the adaptation of microorganisms while still maintaining their potential for outgrowth once a stressful condition is overcome.

Degradation of the microbial stress protectants and chemical chaperones ectoine and hydroxyectoine by a bacterial hydrolase-deacetylase complex

When faced with increased osmolarity in the environment, many bacterial cells accumulate the compatible solute ectoine and its derivative 5-hydroxyectoine. Both compounds are not only potent osmostress protectants, but also serve as effective chemical chaperones stabilizing protein functionality. Ectoines are energy-rich nitrogen and carbon sources that have an ecological impact that shapes microbial communities. Although the biochemistry of ectoine and 5-hydroxyectoine biosynthesis is well understood, our understanding of their catabolism is only rudimentary. Here, we combined biochemical and structural approaches to unravel the core of ectoine and 5-hydroxy-ectoine catabolisms. We show that a conserved enzyme bimodule consisting of the EutD ectoine/5-hydroxyectoine hydrolase and the EutE deacetylase degrades both ectoines. We determined the high-resolution crystal structures of both enzymes, derived from the salt-tolerant bacteria Ruegeria pomeroyi and Halomonas elongata These structures, either in their apo-forms or in forms capturing substrates or intermediates, provided detailed insights into the catalytic cores of the EutD and EutE enzymes. The combined biochemical and structural results indicate that the EutD homodimer opens the pyrimidine ring of ectoine through an unusual covalent intermediate, N-α-2 acetyl-l-2,4-diaminobutyrate (α-ADABA). We found that α-ADABA is then deacetylated by the zinc-dependent EutE monomer into diaminobutyric acid (DABA), which is further catabolized to l-aspartate. We observed that the EutD-EutE bimodule synthesizes exclusively the α-, but not the γ-isomers of ADABA or hydroxy-ADABA. Of note, α-ADABA is known to induce the MocR/GabR-type repressor EnuR, which controls the expression of many ectoine catabolic genes clusters. We conclude that hydroxy-α-ADABA might serve a similar function.

ParB-type DNA Segregation Proteins Are CTP-Dependent Molecular Switches

During cell division, newly replicated DNA is actively segregated to the daughter cells. In most bacteria, this process involves the DNA-binding protein ParB, which condenses the centromeric regions of sister DNA molecules into kinetochore-like structures that recruit the DNA partition ATPase ParA and the prokaroytic SMC/condensin complex. Here, we report the crystal structure of a ParB-like protein (PadC) that emerges to tightly bind the ribonucleotide CTP. The CTP-binding pocket of PadC is conserved in ParB and composed of signature motifs known to be essential for ParB function. We find that ParB indeed interacts with CTP and requires nucleotide binding for DNA condensation in vivo. We further show that CTP-binding modulates the affinity of ParB for centromeric parS sites, whereas parS recognition stimulates its CTPase activity. ParB proteins thus emerge as a new class of CTP-dependent molecular switches that act in concert with ATPases and GTPases to control fundamental cellular functions.

GTP-Dependent FlhF Homodimer Supports Secretion of a Hemolysin in Bacillus cereus

The multidomain (B-NG) protein FlhF, a flagellar biogenesis regulator in several bacteria, is the third paralog of the signal recognition particle (SRP)-GTPases Ffh and FtsY, which are known to drive protein-delivery to the plasma membrane. Previously, we showed that FlhF is required for Bacillus cereus pathogenicity in an insect model of infection, being essential for physiological peritrichous flagellation, for motility, and for the secretion of virulence proteins. Among these proteins, we found that the L₂ component of hemolysin BL, one of the most powerful toxins B. cereus produces, was drastically reduced by the FlhF depletion. Herein, we demonstrate that B. cereus FlhF forms GTP-dependent homodimers in vivo since the replacement of residues critical for their GTP-dependent homodimerization alters this ability. The protein directly or indirectly controls flagellation by affecting flagellin-gene transcription and its overproduction leads to a hyperflagellated phenotype. On the other hand, FlhF does not affect the expression of the L₂-encoding gene (hblC), but physically binds L₂ when in its homodimeric form, recruiting the protein to the plasma membrane for secretion. We additionally show that FlhF overproduction increases L₂ secretion and that the FlhF/L₂ interaction requires the NG domain of FlhF. Our findings demonstrate the peculiar behavior of B. cereus FlhF, which is required for the correct flagellar pattern and acts as SRP-GTPase in the secretion of a bacterial toxin subunit.

The two paralogous kiwellin proteins KWL1 and KWL1-b from maize are structurally related and have overlapping functions in plant defense

Many plant-pathogenic bacteria and fungi deploy effector proteins that down-regulate plant defense responses and reprogram plant metabolism for colonization and survival in planta Kiwellin (KWL) proteins are a widespread family of plant-defense proteins that target these microbial effectors. The KWL1 protein from maize (corn, Zea mays) specifically inhibits the enzymatic activity of the secreted chorismate mutase Cmu1, a virulence-promoting effector of the smut fungus Ustilago maydis. In addition to KWL1, 19 additional KWL paralogs have been identified in maize. Here, we investigated the structure and mechanism of the closest KWL1 homolog, KWL1-b (ZEAMA_GRMZM2G305329). We solved the Cmu1-KWL1-b complex to 2.75 Å resolution, revealing a highly symmetric Cmu1-KWL1-b heterotetramer in which each KWL1-b monomer interacts with a monomer of the Cmu1 homodimer. The structure also revealed that the overall architecture of the heterotetramer is highly similar to that of the previously reported Cmu1-KWL1 complex. We found that upon U. maydis infection of Z. mays, KWL1-b is expressed at significantly lower levels than KWL1 and exhibits differential tissue-specific expression patterns. We also show that KWL1-b inhibits Cmu1 activity similarly to KWL1. We conclude that KWL1 and KWL1-b are part of a redundant defense system that tissue-specifically targets Cmu1. This notion was supported by the observation that both KWL proteins are carbohydrate-binding proteins with distinct and likely tissue-related specificities. Moreover, binding by Cmu1 modulated the carbohydrate-binding properties of both KWLs. These findings indicate that KWL proteins are part of a spatiotemporally coordinated, plant-wide defense response comprising proteins with overlapping activities.

The alarmones (p)ppGpp are part of the heat shock response of Bacillus subtilis

Bacillus subtilis cells are well suited to study how bacteria sense and adapt to proteotoxic stress such as heat, since temperature fluctuations are a major challenge to soil-dwelling bacteria. Here, we show that the alarmones (p)ppGpp, well known second messengers of nutrient starvation, are also involved in the heat stress response as well as the development of thermo-resistance. Upon heat-shock, intracellular levels of (p)ppGpp rise in a rapid but transient manner. The heat-induced (p)ppGpp is primarily produced by the ribosome-associated alarmone synthetase Rel, while the small alarmone synthetases RelP and RelQ seem not to be involved. Furthermore, our study shows that the generated (p)ppGpp pulse primarily acts at the level of translation, and only specific genes are regulated at the transcriptional level. These include the down-regulation of some translation-related genes and the up-regulation of hpf, encoding the ribosome-protecting hibernation-promoting factor. In addition, the alarmones appear to interact with the activity of the stress transcription factor Spx during heat stress. Taken together, our study suggests that (p)ppGpp modulates the translational capacity at elevated temperatures and thereby allows B. subtilis cells to respond to proteotoxic stress, not only by raising the cellular repair capacity, but also by decreasing translation to concurrently reduce the protein load on the cellular protein quality control system.