Structural basis for c-di-AMP-dependent regulation of the bacterial stringent response by receptor protein DarB.

The bacterial second messenger c-di-AMP controls essential cellular processes, including potassium and osmolyte homeostasis. This makes synthesizing enzymes and components involved in c-di-AMP signal transduction intriguing as potential targets for drug development. The c-di-AMP receptor protein DarB of Bacillus subtilis binds the Rel protein and triggers the Rel-dependent stringent response to stress conditions; however, the structural basis for this trigger is unclear. Here, we report crystal structures of DarB in the ligand-free state and of DarB complexed with c-di-AMP, 3'3'-cGAMP, and AMP. We show that DarB forms a homodimer with a parallel, head-to-head assembly of the monomers. We also confirm the DarB dimer binds two cyclic dinucleotide molecules or two AMP molecules; only one adenine of bound c-di-AMP is specifically recognized by DarB, while the second protrudes out of the donut-shaped protein. This enables DarB to bind also 3'3'-cGAMP, as only the adenine fits in the active site. In absence of c-di-AMP, DarB binds to Rel and stimulates (p)ppGpp synthesis, whereas the presence of c-di-AMP abolishes this interaction. Furthermore, the DarB crystal structures reveal no conformational changes upon c-di-AMP binding, leading us to conclude the regulatory function of DarB on Rel must be controlled directly by the bound c-di-AMP. We thus derived a structural model of the DarB-Rel complex via in silico docking, which was validated with mass spectrometric analysis of the chemically crosslinked DarB-Rel complex and mutagenesis studies. We suggest, based on the predicted complex structure, a mechanism of stringent response regulation by c-di-AMP.

A meet-up of two second messengers: the c-di-AMP receptor DarB controls (p)ppGpp synthesis in Bacillus subtilis.

Many bacteria use cyclic di-AMP as a second messenger to control potassium and osmotic homeostasis. In Bacillus subtilis, several c-di-AMP binding proteins and RNA molecules have been identified. Most of these targets play a role in controlling potassium uptake and export. In addition, c-di-AMP binds to two conserved target proteins of unknown function, DarA and DarB, that exclusively consist of the c-di-AMP binding domain. Here, we investigate the function of the c-di-AMP-binding protein DarB in B. subtilis, which consists of two cystathionine-beta synthase (CBS) domains. We use an unbiased search for DarB interaction partners and identify the (p)ppGpp synthetase/hydrolase Rel as a major interaction partner of DarB. (p)ppGpp is another second messenger that is formed upon amino acid starvation and under other stress conditions to stop translation and active metabolism. The interaction between DarB and Rel only takes place if the bacteria grow at very low potassium concentrations and intracellular levels of c-di-AMP are low. We show that c-di-AMP inhibits the binding of DarB to Rel and the DarB-Rel interaction results in the Rel-dependent accumulation of pppGpp. These results link potassium and c-di-AMP signaling to the stringent response and thus to the global control of cellular physiology.

Essentiality of c-di-AMP in Bacillus subtilis: Bypassing mutations converge in potassium and glutamate homeostasis.

In order to adjust to changing environmental conditions, bacteria use nucleotide second messengers to transduce external signals and translate them into a specific cellular response. Cyclic di-adenosine monophosphate (c-di-AMP) is the only known essential nucleotide second messenger. In addition to the well-established role of this second messenger in the control of potassium homeostasis, we observed that glutamate is as toxic as potassium for a c-di-AMP-free strain of the Gram-positive model bacterium Bacillus subtilis. In this work, we isolated suppressor mutants that allow growth of a c-di-AMP-free strain under these toxic conditions. Characterization of glutamate resistant suppressors revealed that they contain pairs of mutations, in most cases affecting glutamate and potassium homeostasis. Among these mutations, several independent mutations affected a novel glutamate transporter, AimA (Amino acid importer A, formerly YbeC). This protein is the major transporter for glutamate and serine in B. subtilis. Unexpectedly, some of the isolated suppressor mutants could suppress glutamate toxicity by a combination of mutations that affect phospholipid biosynthesis and a specific gain-of-function mutation of a mechanosensitive channel of small conductance (YfkC) resulting in the acquisition of a device for glutamate export. Cultivation of the c-di-AMP-free strain on complex medium was an even greater challenge because the amounts of potassium, glutamate, and other osmolytes are substantially higher than in minimal medium. Suppressor mutants viable on complex medium could only be isolated under anaerobic conditions if one of the two c-di-AMP receptor proteins, DarA or DarB, was absent. Also on complex medium, potassium and osmolyte toxicity are the major bottlenecks for the growth of B. subtilis in the absence of c-di-AMP. Our results indicate that the essentiality of c-di-AMP in B. subtilis is caused by the global impact of the second messenger nucleotide on different aspects of cellular physiology.

A Central Role for Magnesium Homeostasis during Adaptation to Osmotic Stress.

Osmotic stress is a significant physical challenge for free-living cells. Cells from all three domains of life maintain viability during osmotic stress by tightly regulating the major cellular osmolyte potassium (K+) and by import or synthesis of compatible solutes. It has been widely established that in response to high salt stress, many bacteria transiently accumulate high levels of K+, leading to bacteriostasis, with growth resuming only when compatible solutes accumulate and K+ levels are restored to biocompatible levels. Using Bacillus subtilis as a model system, we provide evidence that K+ fluxes perturb Mg2+ homeostasis: import of K+ upon osmotic upshift is correlated with Mg2+ efflux, and Mg2+ reimport is critical for adaptation. The transient growth inhibition resulting from hyperosmotic stress is coincident with loss of Mg2+ and a decrease in protein translation. Conversely, the reimport of Mg2+ is a limiting factor during resumption of growth. Furthermore, we show the essential signaling dinucleotide cyclic di-AMP fluctuates dynamically in coordination with Mg2+ and K+ levels, consistent with the proposal that cyclic di-AMP orchestrates the cellular response to osmotic stress. IMPORTANCE Environments with high concentrations of salt or other solutes impose an osmotic stress on cells, ultimately limiting viability by dehydration of the cytosol. A very common cellular response to high osmolarity is to immediately import high levels of potassium ion (K+), which helps prevent dehydration and allows time for the import or synthesis of biocompatible solutes that allow a resumption of growth. Here, using Bacillus subtilis as a model, we demonstrate that concomitant with K+ import there is a large reduction in intracellular magnesium (Mg2+) mediated by specific efflux pumps. Further, it is the reimport of Mg2+ that is rate-limiting for the resumption of growth. These coordinated fluxes of K+ and Mg2+ are orchestrated by cyclic-di-AMP, an essential second messenger in Firmicutes. These findings amend the conventional model for osmoadaptation and reveal that Mg2+ limitation is the proximal cause of the bacteriostasis that precedes resumption of growth.

Sustained Control of Pyruvate Carboxylase by the Essential Second Messenger Cyclic di-AMP in Bacillus subtilis.

In Bacillus subtilis and other Gram-positive bacteria, cyclic di-AMP is an essential second messenger that signals potassium availability by binding to a variety of proteins. In some bacteria, c-di-AMP also binds to the pyruvate carboxylase to inhibit its activity. We have discovered that in B. subtilis the c-di-AMP target protein DarB, rather than c-di-AMP itself, specifically binds to pyruvate carboxylase both in vivo and in vitro. This interaction stimulates the activity of the enzyme, as demonstrated by in vitro enzyme assays and in vivo metabolite determinations. Both the interaction and the activation of enzyme activity require apo-DarB and are inhibited by c-di-AMP. Under conditions of potassium starvation and corresponding low c-di-AMP levels, the demand for citric acid cycle intermediates is increased. Apo-DarB helps to replenish the cycle by activating both pyruvate carboxylase gene expression and enzymatic activity via triggering the stringent response as a result of its interaction with the (p)ppGpp synthetase Rel and by direct interaction with the enzyme, respectively. IMPORTANCE If bacteria experience a starvation for potassium, by far the most abundant metal ion in every living cell, they have to activate high-affinity potassium transporters, switch off growth activities such as translation and transcription of many genes or replication, and redirect the metabolism in a way that the most essential functions of potassium can be taken over by metabolites. Importantly, potassium starvation triggers a need for glutamate-derived amino acids. In many bacteria, the responses to changing potassium availability are orchestrated by a nucleotide second messenger, cyclic di-AMP. c-di-AMP binds to factors involved directly in potassium homeostasis and to dedicated signal transduction proteins. Here, we demonstrate that in the Gram-positive model organism Bacillus subtilis, the c-di-AMP receptor protein DarB can bind to and, thus, activate pyruvate carboxylase, the enzyme responsible for replenishing the citric acid cycle. This interaction takes place under conditions of potassium starvation if DarB is present in the apo form and the cells are in need of glutamate. Thus, DarB links potassium availability to the control of central metabolism.

Cyclic di-AMP Signaling in Bacteria.

The second messenger molecule cyclic di-AMP (c-di-AMP) is formed by many bacteria and archaea. In many species that produce c-di-AMP, this second messenger is essential for viability on rich medium. Recent research has demonstrated that c-di-AMP binds to a large number of proteins and riboswitches, which are often involved in potassium and osmotic homeostasis. c-di-AMP becomes dispensable if the bacteria are cultivated on minimal media with low concentrations of osmotically active compounds. Thus, the essentiality of c-di-AMP does not result from an interaction with a single essential target but rather from the multilevel control of complex homeostatic processes. This review summarizes current knowledge on the homeostasis of c-di-AMP and its function(s) in the control of cellular processes.

Qualidade de Vida no Trabalho em um Instituto de Pesquisa Brasileiro / Quality of Work Life in a Brazilian Research Institute / Calidad de Vida Laboral en un Instituto de Investigación Brasileño

O objetivo da pesquisa foi analisar a percepção de QVT dos trabalhadores de um instituto de pesquisa brasileiro. A base teórico-metodológica para o estudo é a Ergonomia da Atividade Aplicada à Qualidade de Vida no Trabalho. Participaram da pesquisa 275 respondentes. Para a coleta de dados, utilizou-se o Inventário de Avaliação de Qualidade de Vida no Trabalho, disponibilizado em versão digital para todos os trabalhadores da organização. O tratamento dos dados foi realizado por meio do software estatístico SPSS. Os principais resultados indicam que: os trabalhadores da organização percebem bem-estar no trabalho moderado e que fatores "organização do trabalho" e "reconhecimento e crescimento profissional" apontam para necessidade de mudanças. Os resultados obtidos forneceram subsídios fundamentais para o instituto formular o programa de QVT da organização.

The objective of this research was to analyze how Brazilian Research Insti-tute workers perceive QWL. The theoretical and methodological basis for the study was the Activity Ergonomics Applied to Quality of Work Life. For this purpose, 275 persons participated in the survey. To collect the data, we used the Inventory Assessment of Quality of Work Life, available in digital version for all workers of the organization. Data analysis was carried out using SPSS software. The main results indicated that: workers perceive organizational well-being at moderate level and that the factors "work organization" and "recognition and professional growth" indicate the need for changes. The results provided crucial information for the institute, so that the QWL program of the organization could be reformulated.

El objetivo de la investigación fue analizar la percepción de CVL de los trabaja-dores de un instituto de investigación brasileño. La base teórica y metodológica para el estudio es la Ergonomía de la Actividad Aplicada a la de Calidad de Vida Laboral. Participaron de esta investigación 275 personas. Para recopilar los datos, se utilizó el Inventario de Evaluación de la Calidad de Vida Laboral, disponible en versión digital para todos los empleados de la organización. El análisis de datos se realizó a través del programa SPSS. Los principales resul-tados indicaron que: los trabajadores de la organización perciben bienestar moderado y que factores como "organización del trabajo" y "reconocimiento y crecimiento profesional" apuntan la necesidad de cambios. Los resultados proporcionaron informaciones fundamentales para que el instituto formule el programa de CVL de la organización.

Sustained sensing in potassium homeostasis: Cyclic di-AMP controls potassium uptake by KimA at the levels of expression and activity.

The signaling nucleotide cyclic di-AMP (c-di-AMP) is the only known essential second messenger in bacteria. Recently, c-di-AMP has been identified as being essential for controlling potassium uptake in the model organism Bacillus subtilis and several other bacteria. A B. subtilis strain lacking c-di-AMP is not viable at high potassium concentrations, unless the bacteria acquire suppressor mutations. In this study, we isolated such suppressor mutants and found mutations that reduced the activities of the potassium transporters KtrCD and KimA. Although c-di-AMP-mediated control of KtrCD has previously been demonstrated, it is unknown how c-di-AMP affects KimA activity. Using the DRaCALA screening assay, we tested for any interactions of KimA and other potential target proteins in B. subtilis with c-di-AMP. This assay identified KimA, as well as the K+/H+ antiporter KhtT, the potassium exporter CpaA (YjbQ), the osmoprotectant transporter subunit OpuCA, the primary Mg2+ importer MgtE, and DarB (YkuL), a protein of unknown function, as bona fide c-di-AMP-binding proteins. Further, binding of c-di-AMP to KimA inhibited potassium uptake. Our results indicate that c-di-AMP controls KimA-mediated potassium transport at both kimA gene expression and KimA activity levels. Moreover, the discovery that potassium exporters are c-di-AMP targets indicates that this second messenger controls potassium homeostasis in B. subtilis at a global level by binding to riboswitches and to different classes of transport proteins involved in potassium uptake and export.

Influence of amifostine on toxicity of CHOP in elderly patients with aggressive non-Hodgkin's lymphoma--a phase II study.

Due to concerns about toxicity, many elderly patients with aggressive non-Hodgkin's lymphoma (NHL) are not considered candidates for standard chemotherapy with cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP). The cytoprotective agent amifostine has the potential to reduce toxicity when added to chemotherapy. The purpose of the current study was to examine the toxicity of CHOP combined with amifostine in elderly patients with aggressive NHL. A prospective phase II study was performed in patients aged 60 years and older. Patients with stage I/II disease received 4 cycles of CHOP followed by involved-field irradiation. Patients with stage III/IV received 6-8 cycles of CHOP. Amifostine (740 mg/m(2)) was administered as a 15-min i.v. infusion immediately before chemotherapy. Forty-one (median age 69.5 years, range 60-87) of 49 consecutive previously untreated patients, aged 60 years and older, with aggressive NHL seen in our center were included in the study. Twenty-one patients had stage I/II disease and 20 had stage III/IV disease. The patients received a total of 207 cycles of amifostine-CHOP. Infusion of amifostine caused mild to moderate transient side effects, including a drop of systolic blood pressure >20 mmHg in 54 cycles and nausea/vomiting in 36 cycles. Hematotoxicity of CHOP consisted of leukopenia grade 4 in only 15.4% of cycles. There were two cases of grade 3 anemia. No thrombocytopenia higher than grade 2 occurred. Febrile neutropenia was rare, occurring in 4.3% of cycles. One patient died after the first CHOP administration because of anthracycline-related acute cardiomyopathy (corresponding to a toxic death rate of 2.4%). The complete response rates were 85 and 75% in stage I/II and stage III/IV patients, respectively. After median follow-up of 33 months (range 17-50 months) the median overall survival was not reached in patients with stage I/II and was found to be 32 months in patients with stage III/IV. At 2 years, 76% of patients with stage I/II and 70% with stage III/IV were alive. Twelve of the 15 patients who died were aged older than 70. Amifostine pre-treatment was associated with a low toxicity of CHOP in elderly patients with aggressive NHL treated with curative intent. Treatment outcomes appeared not to be impaired by the addition of amifostine to CHOP. This schedule merits further testing in a randomized trial.