How the central domain of dystrophin acts to bridge F-actin to sarcolemmal lipids.

Dystrophin is a large intracellular protein that prevents sarcolemmal ruptures by providing a mechanical link between the intracellular actin cytoskeleton and the transmembrane dystroglycan complex. Dystrophin deficiency leads to the severe muscle wasting disease Duchenne Muscular Dystrophy and the milder allelic variant, Becker Muscular Dystrophy (DMD and BMD). Previous work has shown that concomitant interaction of the actin binding domain 2 (ABD2) comprising spectrin like repeats 11 to 15 (R11-15) of the central domain of dystrophin, with both actin and membrane lipids, can greatly increase membrane stiffness. Based on a combination of SAXS and SANS measurements, mass spectrometry analysis of cross-linked complexes and interactive low-resolution simulations, we explored in vitro the molecular properties of dystrophin that allow the formation of ABD2-F-actin and ABD2-membrane model complexes. In dystrophin we identified two subdomains interacting with F-actin, one located in R11 and a neighbouring region in R12 and another one in R15, while a single lipid binding domain was identified at the C-terminal end of R12. Relative orientations of the dystrophin central domain with F-actin and a membrane model were obtained from docking simulation under experimental constraints. SAXS-based models were then built for an extended central subdomain from R4 to R19, including ABD2. Overall results are compatible with a potential F-actin/dystrophin/membrane lipids ternary complex. Our description of this selected part of the dystrophin associated complex bridging muscle cell membrane and cytoskeleton opens the way to a better understanding of how cell muscle scaffolding is maintained through this essential protein.

Dystrophin's central domain forms a complex filament that becomes disorganized by in-frame deletions.

Dystrophin, encoded by the DMD gene, is critical for maintaining plasma membrane integrity during muscle contraction events. Mutations in the DMD gene disrupting the reading frame prevent dystrophin production and result in severe Duchenne muscular dystrophy (DMD); in-frame internal deletions allow production of partly functional internally deleted dystrophin and result in less severe Becker muscular dystrophy (BMD). Many known BMD deletions occur in dystrophin's central domain, generally considered to be a monotonous rod-shaped domain based on the knowledge of spectrin family proteins. However, the effects caused by these deletions, ranging from asymptomatic to severe BMD, argue against the central domain serving only as a featureless scaffold. We undertook structural studies combining small-angle X-ray scattering and molecular modeling in an effort to uncover the structure of the central domain, as dystrophin has been refractory to characterization. We show that this domain appears to be a tortuous and complex filament that is profoundly disorganized by the most severe BMD deletion (loss of exons 45-47). Despite the preservation of large parts of the binding site for neuronal nitric oxide synthase (nNOS) in this deletion, computational approaches failed to recreate the association of dystrophin with nNOS. This observation is in agreement with a strong decrease of nNOS immunolocalization in muscle biopsies, a parameter related to the severity of BMD phenotypes. The structural description of the whole dystrophin central domain we present here is a first necessary step to improve the design of microdystrophin constructs toward the goal of a successful gene therapy for DMD.

Human Dystrophin Structural Changes upon Binding to Anionic Membrane Lipids.

Scaffolding proteins play important roles in supporting the plasma membrane (sarcolemma) of muscle cells. Among them, dystrophin strengthens the sarcolemma through protein-lipid interactions, and its absence due to gene mutations leads to the severe Duchenne muscular dystrophy. Most of the dystrophin protein consists of a central domain made of 24 spectrin-like coiled-coil repeats (R). Using small angle neutron scattering (SANS) and the contrast variation technique, we specifically probed the structure of the three first consecutive repeats 1-3 (R1-3), a part of dystrophin known to physiologically interact with membrane lipids. R1-3 free in solution was compared to its structure adopted in the presence of phospholipid-based bicelles. SANS data for the protein/lipid complexes were obtained with contrast-matched bicelles under various phospholipid compositions to probe the role of electrostatic interactions. When bound to anionic bicelles, large modifications of the protein three-dimensional structure were detected, as revealed by a significant increase of the protein gyration radius from 42 ± 1 to 60 ± 4 Å. R1-3/anionic bicelle complexes were further analyzed by coarse-grained molecular dynamics simulations. From these studies, we report an all-atom model of R1-3 that highlights the opening of the R1 coiled-coil repeat when bound to the membrane lipids. This model is totally in agreement with SANS and click chemistry/mass spectrometry data. We conclude that the sarcolemma membrane anchoring that occurs during the contraction/elongation process of muscles could be ensured by this coiled-coil opening. Therefore, understanding these structural changes may help in the design of rationalized shortened dystrophins for gene therapy. Finally, our strategy opens up new possibilities for structure determination of peripheral and integral membrane proteins not compatible with different high-resolution structural methods.

Becker muscular dystrophy severity is linked to the structure of dystrophin.

In-frame exon deletions of the Duchenne muscular dystrophy (DMD) gene produce internally truncated proteins that typically lead to Becker muscular dystrophy (BMD), a milder allelic disorder of DMD. We hypothesized that differences in the structure of mutant dystrophin may be responsible for the clinical heterogeneity observed in Becker patients and we studied four prevalent in-frame exon deletions, i.e. Δ45-47, Δ45-48, Δ45-49 and Δ45-51. Molecular homology modelling revealed that the proteins corresponding to deletions Δ45-48 and Δ45-51 displayed a similar structure (hybrid repeat) than the wild-type dystrophin, whereas deletions Δ45-47 and Δ45-49 lead to proteins with an unrelated structure (fractional repeat). All four proteins in vitro expressed in a fragment encoding repeats 16-21 were folded in α-helices and remained highly stable. Refolding dynamics were slowed and molecular surface hydrophobicity were higher in fractional repeat containing Δ45-47 and Δ45-49 deletions compared with hybrid repeat containing Δ45-48 and Δ45-51 deletions. By retrospectively collecting data for a series of French BMD patients, we showed that the age of dilated cardiomyopathy (DCM) onset was delayed by 11 and 14 years in Δ45-48 and Δ45-49 compared with Δ45-47 patients, respectively. A clear trend toward earlier wheelchair dependency (minimum of 11 years) was also observed in Δ45-47 and Δ45-49 patients compared with Δ45-48 patients. Muscle dystrophin levels were moderately reduced in most patients without clear correlation with the deletion type. Disease progression in BMD patients appears to be dependent on the deletion itself and associated with a specific structure of dystrophin at the deletion site.

Novel mutation in spectrin-like repeat 1 of dystrophin central domain causes protein misfolding and mild Becker muscular dystrophy.

Mutations in the dystrophin gene without disruption of the reading frame often lead to Becker muscular dystrophy, but a genotype/phenotype correlation is difficult to establish. Amino acid substitutions may disrupt binding capacities of dystrophin and have a major impact on the functionality of this protein. We have identified two brothers (ages 8 and 10 years) with very mild proximal weakness, recurrent abdominal pain, and moderately elevated serum creatine kinase levels. Gene sequencing revealed a novel mutation in exon 11 of the dystrophin gene (c.1280T>C) leading to a L427P amino acid substitution in repeat 1 of the central rod domain. Immunostaining of skeletal muscle showed weak staining of the dystrophin region encoded by exons 7 and 8 corresponding to the end of the actin-binding domain 1 and the N-terminal part of hinge 1. Spectrofluorescence and circular dichroism analysis of the domain repeat 1-2 (R1-2) revealed partial misfolding of the L427P mutated protein as well as a reduced refolding rate after denaturation. Based on computational homology models of the wild-type and mutated R1-2, a molecular dynamics study showed an alteration in the flexibility of the structure, which also strongly affects the conformational space available in the N-terminal region of the fragment. Our results suggest that this missense mutation hinders the dynamic properties of the entire N-terminal region of dystrophin.

The LC3B FRET biosensor monitors the modes of action of ATG4B during autophagy in living cells.

Although several mechanisms of macroautophagy/autophagy have been dissected in the last decade, following this pathway in real time remains challenging. Among the early events leading to its activation, the ATG4B protease primes the key autophagy player MAP1LC3B/LC3B. Given the lack of reporters to follow this event in living cells, we developed a Förster's resonance energy transfer (FRET) biosensor responding to the priming of LC3B by ATG4B. The biosensor was generated by flanking LC3B within a pH-resistant donor-acceptor FRET pair, Aquamarine-tdLanYFP. We here showed that the biosensor has a dual readout. First, FRET indicates the priming of LC3B by ATG4B and the resolution of the FRET image makes it possible to characterize the spatial heterogeneity of the priming activity. Second, quantifying the number of Aquamarine-LC3B puncta determines the degree of autophagy activation. We then showed that there are pools of unprimed LC3B upon ATG4B downregulation, and the priming of the biosensor is abolished in ATG4B knockout cells. The lack of priming can be rescued with the wild-type ATG4B or with the partially active W142A mutant, but not with the catalytically dead C74S mutant. Moreover, we screened for commercially-available ATG4B inhibitors, and illustrated their differential mode of action by implementing a spatially-resolved, broad-to-sensitive analysis pipeline combining FRET and the quantification of autophagic puncta. Finally, we uncovered the CDK1-dependent regulation of the ATG4B-LC3B axis at mitosis. Therefore, the LC3B FRET biosensor paves the way for a highly-quantitative monitoring of the ATG4B activity in living cells and in real time, with unprecedented spatiotemporal resolution.Abbreviations: Aqua: aquamarine; ATG: autophagy related; AURKA: aurora kinase A; BafA1: bafilomycin A1; CDK1: cyclin dependent kinase 1; DKO: double knockout; FLIM: fluorescence lifetime imaging microscopy; FP: fluorescence protein; FRET: Förster's resonance energy transfer; GABARAP: GABA type A receptor-associated protein; HBSS: Hanks' balanced salt solution; KO: knockout; LAMP2: lysosomal associated membrane protein 2; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; NSC: NSC 185058; PE: phosphatidylethanolamine; SKO: single knockout; TKO: triple knockout; ULK1: unc-51 like autophagy activating kinase 1; WT: wild-type; ZPCK: Z-L-phe chloromethyl ketone.

Spectrin-like repeats 11-15 of human dystrophin show adaptations to a lipidic environment.

Dystrophin is essential to skeletal muscle function and confers resistance to the sarcolemma by interacting with cytoskeleton and membrane. In the present work, we characterized the behavior of dystrophin 11-15 (DYS R11-15), five spectrin-like repeats from the central domain of human dystrophin, with lipids. DYS R11-15 displays an amphiphilic character at the liquid/air interface while maintaining its secondary α-helical structure. The interaction of DYS R11-15 with small unilamellar vesicles (SUVs) depends on the lipid nature, which is not the case with large unilamellar vesicles (LUVs). In addition, switching from anionic SUVs to anionic LUVs suggests the lipid packing as a crucial factor for the interaction of protein and lipid. The monolayer model and the modulation of surface pressure aim to mimic the muscle at work (i.e. dynamic changes of muscle membrane during contraction and relaxation) (high and low surface pressure). Strikingly, the lateral pressure modifies the protein organization. Increasing the lateral pressure leads the proteins to be organized in a regular network. Nevertheless, a different protein conformation after its binding to monolayer is revealed by trypsin proteolysis. Label-free quantification by nano-LC/MS/MS allowed identification of the helices in repeats 12 and 13 involved in the interaction with anionic SUVs. These results, combined with our previous studies, indicate that DYS R11-15 constitutes the only part of dystrophin that interacts with anionic as well as zwitterionic lipids and adapts its interaction and organization depending on lipid packing and lipid nature. We provide strong experimental evidence for a physiological role of the central domain of dystrophin in sarcolemma scaffolding through modulation of lipid-protein interactions.

New insights into chlorophyll-WSCP (water-soluble chlorophyll proteins) interactions : The case study of BnD22 (Brassica napus drought-induced 22 kDa).

The water-soluble chlorophyll-proteins (WSCP) of class II from Brassicaceae are non-photosynthetic proteins that bind chlorophylls (Chls) and chlorophyll derivatives. Their physiological roles, biochemical functions and mode of action are still unclear. It is assumed that the WSCPs have a protection function against Chl photodamage during stressful conditions. WSCPs are subdivided into class IIA and class IIB according to their apparent Chla/b binding ratio. Although their Chla/Chlb binding selectivity has been partly characterized, their Chl affinities are not yet precisely defined. For instance, WSCPs IIA do not show any Chl binding preference while WSCPs IIB have greater affinity to Chlb. In this study, we present a novel method for assessment of Chl binding to WSCPs based on the differences of Chl photobleaching rates in a large range of Chl/protein ratios. The protein we have chosen to study WSCP is BnD22, a WSCP IIA induced in the leaves of Brassica napus under water deficit. BnD22 formed oligomeric complexes upon binding to Chla and/or Chlb allowing a protective effect against photodamage. The binding constants indicate that BnD22 binds with high affinity the Chls and with a strong selectivity to Chla. Moreover, dependending of Chl/protein ratio upon reconstitution, two distinct binding events were detected resulting from difference of Chl stoichiometry inside oligomeric complexes.

Mitochondrial Aurora kinase A induces mitophagy by interacting with MAP1LC3 and Prohibitin 2.

Epithelial and haematologic tumours often show the overexpression of the serine/threonine kinase AURKA. Recently, AURKA was shown to localise at mitochondria, where it regulates mitochondrial dynamics and ATP production. Here we define the molecular mechanisms of AURKA in regulating mitochondrial turnover by mitophagy. AURKA triggers the degradation of Inner Mitochondrial Membrane/matrix proteins by interacting with core components of the autophagy pathway. On the inner mitochondrial membrane, the kinase forms a tripartite complex with MAP1LC3 and the mitophagy receptor PHB2, which triggers mitophagy in a PARK2/Parkin-independent manner. The formation of the tripartite complex is induced by the phosphorylation of PHB2 on Ser39, which is required for MAP1LC3 to interact with PHB2. Last, treatment with the PHB2 ligand xanthohumol blocks AURKA-induced mitophagy by destabilising the tripartite complex and restores normal ATP production levels. Altogether, these data provide evidence for a role of AURKA in promoting mitophagy through the interaction with PHB2 and MAP1LC3. This work paves the way to the use of function-specific pharmacological inhibitors to counteract the effects of the overexpression of AURKA in cancer.

Fine mapping of hydrophobic contacts reassesses the organization of the first three dystrophin coiled-coil repeats.

Coiled-coil domain is a structural motif found in proteins crucial for achievement of central biological processes, such as cellular cohesion or neuro-transmission. The coiled-coil fold consists of alpha-helices bundle that can be repeated to form larger filament. Hydrophobic residues, distributed following a regular seven-residues' pattern, named heptad pattern, are commonly admitted to be essential for the formation and the stability of canonical coiled-coil repeats. Here we investigated the first three coiled-coil repeats (R1-R3) of the central domain of dystrophin, a scaffolding protein in muscle cells whose deficiency leads to Duchenne and Becker Muscular Dystrophies. By an atomic description of the hydrophobic interactions, we highlighted (i) that coiled-coil filament conformational changes are associated to specific patterns of inter-helices hydrophobic contacts, (ii) that inter-repeat hydrophobic interactions determine the behavior of linker regions including filament kinks, and (iii) that a non-strict conservation of the heptad patterns is leading to a relative plasticity of the dystrophin coiled-coil repeats. These structural features and modulations of the coiled-coil fold could better explain the mechanical properties of the central domain of dystrophin. This contribution to the understanding of the structure-function relationship of dystrophin, and especially of the R1-R3 fragment frequently used in the design of protein for gene therapies, should help in the improvement of the strategies for the cure of muscular dystrophies.

OxyGene: an innovative platform for investigating oxidative-response genes in whole prokaryotic genomes.

BACKGROUND: Oxidative stress is a common stress encountered by living organisms and is due to an imbalance between intracellular reactive oxygen and nitrogen species (ROS, RNS) and cellular antioxidant defence. To defend themselves against ROS/RNS, bacteria possess a subsystem of detoxification enzymes, which are classified with regard to their substrates. To identify such enzymes in prokaryotic genomes, different approaches based on similarity, enzyme profiles or patterns exist. Unfortunately, several problems persist in the annotation, classification and naming of these enzymes due mainly to some erroneous entries in databases, mistake propagation, absence of updating and disparity in function description. DESCRIPTION: In order to improve the current annotation of oxidative stress subsystems, an innovative platform named OxyGene has been developed. It integrates an original database called OxyDB, holding thoroughly tested anchor-based signatures associated to subfamilies of oxidative stress enzymes, and a new anchor-driven annotator, for ab initio detection of ROS/RNS response genes. All complete Bacterial and Archaeal genomes have been re-annotated, and the results stored in the OxyGene repository can be interrogated via a Graphical User Interface. CONCLUSION: OxyGene enables the exploration and comparative analysis of enzymes belonging to 37 detoxification subclasses in 664 microbial genomes. It proposes a new classification that improves both the ontology and the annotation of the detoxification subsystems in prokaryotic whole genomes, while discovering new ORFs and attributing precise function to hypothetical annotated proteins. OxyGene is freely available at: http://www.umr6026.univ-rennes1.fr/english/home/research/basic/software.

Analyzing stochastic transcription to elucidate the nucleoid's organization.

BACKGROUND: The processes of gene transcription, translation, as well as the reactions taking place between gene products, are subject to stochastic fluctuations. These stochastic events are being increasingly examined as it emerges that they can be crucial in the cell's survival. In a previous study we had examined the transcription patterns of two bacterial species (Escherichia coli and Bacillus subtilis) to elucidate the nucleoid's organization. The basic idea is that genes that share transcription patterns, must share some sort of spatial relationship, even if they are not close to each other on the chromosome. We had found that picking any gene at random, its transcription will be correlated with genes at well-defined short - as well as long-range distances, leaving the explanation of the latter an open question. In this paper we study the transcription correlations when the only transcription taking place is stochastic, in other words, no active or "deterministic" transcription takes place. To this purpose we use transcription data of Sinorhizobium meliloti. RESULTS: Even when only stochastic transcription takes place, the co-expression of genes varies as a function of the distance between genes: we observe again the short-range as well as the regular, long-range correlation patterns. CONCLUSION: We explain these latter with a model based on the physical constraints acting on the DNA, forcing it into a conformation of groups of a few successive large and transcribed loops, which are evenly spaced along the chromosome and separated by small, non-transcribed loops. We discuss the question about the link between shared transcription patterns and physiological relationship and come to the conclusion that when genes are distantly placed along the chromosome, the transcription correlation does not imply a physiological relationship.

Identification of chromosomal alpha-proteobacterial small RNAs by comparative genome analysis and detection in Sinorhizobium meliloti strain 1021.

BACKGROUND: Small untranslated RNAs (sRNAs) seem to be far more abundant than previously believed. The number of sRNAs confirmed in E. coli through various approaches is above 70, with several hundred more sRNA candidate genes under biological validation. Although the total number of sRNAs in any one species is still unclear, their importance in cellular processes has been established. However, unlike protein genes, no simple feature enables the prediction of the location of the corresponding sequences in genomes. Several approaches, of variable usefulness, to identify genomic sequences encoding sRNA have been described in recent years. RESULTS: We used a combination of in silico comparative genomics and microarray-based transcriptional profiling. This approach to screening identified ~60 intergenic regions conserved between Sinorhizobium meliloti and related members of the alpha-proteobacteria sub-group 2. Of these, 14 appear to correspond to novel non-coding sRNAs and three are putative peptide-coding or 5' UTR RNAs (ORF smaller than 100 aa). The expression of each of these new small RNA genes was confirmed by Northern blot hybridization. CONCLUSION: Small non coding RNA (sra) genes can be found in the intergenic regions of alpha-proteobacteria genomes. Some of these sra genes are only present in S. meliloti, sometimes in genomic islands; homologues of others are present in related genomes including those of the pathogens Brucella and Agrobacterium.

Characterization and expression patterns of Sinorhizobium meliloti tmRNA (ssrA).

tmRNA (ssrA) in Sinorhizobium meliloti is a small RNA annotated by homology with the Bradyrhizobium japonicum sra molecule. Here, this molecule is described in Sinorhizobium meliloti as a model for such molecules in Alphaproteobacteria subgroup-2. Northern blot analysis and mapping of both 5' and 3' ends of this tmRNA allow the identification of two pieces: a 214 nt mRNA-like domain and an 82 nt tRNA-like domain, both highly stable, whereas the premature form is unstable. Transcriptional studies reveal that Sinorhizobium meliloti tmRNA is mainly expressed during growth resumption, replication initiation and various stress responses.

Transcarboxylase mRNA: a marker which evidences P. freudenreichii survival and metabolic activity during its transit in the human gut.

Dairy propionibacteria have recently been considered as probiotics which may beneficially modulate the intestinal ecosystem. However, appropriate vectors (food matrices containing the probiotic) which preserve their viability and offer good tolerance towards digestive stresses need to be developed. In addition, the development of efficient non-invasive methods which specifically monitor Propionibacterium freudenreichii concentration and activity within the human gut is required. To address this latter need, an enzyme involved in propionic fermentation, transcarboxylase, was evaluated in this study as molecular marker in P. freudenreichii. In vitro, the three transcarboxylase subunits were shown to be encoded by an operon and their expression regulated. It occurred during propionic fermentation, ceased in starved cells and was not affected by digestive stresses. The 5S subunit gene of transcarboxylase allowed specific detection of P. freudenreichii by real time PCR in the complex human faecal microbiota. A dairy vector harbouring P. freudenreichii was developed and afforded elevated probiotic faecal concentrations in humans. In vivo, this PCR method allowed rapid quantification of faecal P. freudenreichii in agreement with the cultural method (cfu counting). Moreover, real time Reverse Transcription (RT) -PCR evidenced transcription of the 5S subunit gene during transit through the human digestive tract. This work constitutes a methodological advance for survival and activity evaluation in human trials of the probiotics belonging to the P. freudenreichii species. It strongly suggests that this bacterium not only survives but remains metabolically active in the human gut.

Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis.

Symbiosis between legumes and Rhizobium bacteria leads to the formation of root nodules where bacteria in the infected plant cells are converted into nitrogen-fixing bacteroids. Nodules with a persistent meristem are indeterminate, whereas nodules without meristem are determinate. The symbiotic plant cells in both nodule types are polyploid because of several cycles of endoreduplication (genome replication without mitosis and cytokinesis) and grow consequently to extreme sizes. Here we demonstrate that differentiation of bacteroids in indeterminate nodules of Medicago and related legumes from the galegoid clade shows remarkable similarity to host cell differentiation. During bacteroid maturation, repeated DNA replication without cytokinesis results in extensive amplification of the entire bacterial genome and elongation of bacteria. This finding reveals a positive correlation in prokaryotes between DNA content and cell size, similar to that in eukaryotes. These polyploid bacteroids are metabolically functional but display increased membrane permeability and are nonviable, because they lose their ability to resume growth. In contrast, bacteroids in determinate nodules of the nongalegoid legumes lotus and bean are comparable to free-living bacteria in their genomic DNA content, cell size, and viability. Using recombinant Rhizobium strains nodulating both legume types, we show that bacteroid differentiation is controlled by the host plant. Plant factors present in nodules of galegoid legumes but absent from nodules of nongalegoid legumes block bacterial cell division and trigger endoreduplication cycles, thereby forcing the endosymbionts toward a terminally differentiated state. Hence, Medicago and related legumes have evolved a mechanism to dominate the symbiosis.

Smc01944, a secreted peroxidase induced by oxidative stresses in Sinorhizobium meliloti 1021.

Sequencing of the Sinorhizobium meliloti strain 1021 genome led to the detection of 6204 open reading frames, 41 % of which have no hypothetical function. To help annotate this genome, a transcriptome analysis was carried out with a dedicated microarray consisting of 146 genes belonging to three different classes: (i) no hypothetical function; (ii) potentially involved in oxidative stress responses; (iii) known to participate in oxidative stress responses (e.g. catalase and superoxide dismutase genes). This transcriptome analysis, together with biological experiments and in silico investigations, identified new genes induced by exogenous H(2)O(2). The smc01944 gene was the most strongly induced: quantitative PCR showed that the amount of smc01944 mRNA increased 50-fold following the addition of 10 mM H(2)O(2), whereas the amount of katA mRNA (encoding a catalase) only increased 10-fold. Smc01944 is a non-haem chloroperoxidase (Cpo). The only member of this family to have been so far characterized is encoded by prxC of Pseudomonas fluorescens. Unexpectedly, the NH(2)-terminus of Smc01944 includes a signal peptide and Smc01944 is secreted into the supernatant. Interestingly, smc01944 is preceded by smc01945, encoding an OhrR-like regulator (MarR family). Thus, Smc01944 is the first exported Cpo encoded by a gene possibly regulated by an OhrR regulator. It was also shown that smc01944 is induced by t-butyl and cumene hydroperoxides but only slightly by menadione. The study of Smc01944 described in this work showed that the oxidative stress response of S. meliloti seems to differ from that of other bacteria characterized to date.