Impaired cell envelope resulting from arcA mutation largely accounts for enhanced sensitivity to hydrogen peroxide in Shewanella oneidensis

The ArcAB Two-Component System: Function in Metabolism, Redox Control, and Infection

ArcAB, also known as the Arc system, is a member of the two-component system family of bacterial transcriptional regulators and is composed of sensor kinase ArcB and response regulator ArcA. In this review, we describe the structure and function of these proteins and assess the state of the literature regarding ArcAB as a sensor of oxygen consumption. The bacterial quinone pool is the primary modulator of ArcAB activity, but questions remain for how this regulation occurs. This review highlights the role of quinones and their oxidation state in activating and deactivating ArcB and compares competing models of the regulatory mechanism. The cellular processes linked to ArcAB regulation of central metabolic pathways and potential interactions of the Arc system with other regulatory systems are also reviewed. Recent evidence for the function of ArcAB under aerobic conditions is challenging the long-standing characterization of this system as strictly an anaerobic global regulator, and the support for additional ArcAB functionality in this context is explored. Lastly, ArcAB-controlled cellular processes with relevance to infection are assessed.

Shewanella oneidensis arcA Mutation Impairs Aerobic Growth Mainly by Compromising Translation

Arc (anoxic redox control), one of the most intensely investigated two-component regulatory systems in γ-proteobacteria, plays a major role in mediating the metabolic transition from aerobiosis to anaerobiosis. In Shewanella oneidensis, a research model for respiratory versatility, Arc is crucial for aerobic growth. However, how this occurs remains largely unknown. In this study, we demonstrated that the loss of the response regulator ArcA distorts the correlation between transcription and translation by inhibiting the ribosome biosynthesis. This effect largely underlies the growth defect because it concurs with the effect of chloramphenicol, which impairs translation. Reduced transcription of ArcA-dependent ribosomal protein S1 appears to have a significant impact on ribosome assembly. We further show that the lowered translation efficiency is not accountable for the envelope defect, another major defect resulting from the ArcA loss. Overall, our results suggest that although the arcA mutation impairs growth through multi-fold complex impacts in physiology, the reduced translation efficacy appears to be a major cause for the phenotype, demonstrating that Arc is a primary system that coordinates proteomic resources with metabolism in S. oneidensis.

Lipopolysaccharide Transport System Links Physiological Roles of σE and ArcA in the Cell Envelope Biogenesis in Shewanella oneidensis

The bacterial cell envelope is not only a protective structure that surrounds the cytoplasm but also the place where a myriad of biological processes take place. This multilayered complex is particularly important for electroactive bacteria such as Shewanella oneidensis, as it generally hosts branched electron transport chains and numerous reductases for extracellular respiration. However, little is known about how the integrity of the cell envelope is established and maintained in these bacteria. By tracing the synthetic lethal effect of Arc two-component system and σ^E in S. oneidensis, in this study, we identified the lipopolysaccharide transport (Lpt) system as the determining factor. Both Arc and σ^E, by regulating transcription of lptFG and lptD, respectively, are required for the Lpt system to function properly. The ArcA loss results in an LptFG shortage that triggers activation of σ^E and leads to LptD overproduction. LptFG and LptD at abnormal levels cause a defect in the lipopolysaccharide (LPS) transport, leading to cell death unless σ^E-dependent envelope stress response is in place. Overall, our report reveals for the first time that Arc works together with σ^E to maintain the integrity of the S. oneidensis cell envelope by participating in the regulation of the LPS transport system.

IMPORTANCE Arc is a well-characterized global regulatory system that modulates cellular respiration by responding to changes in the redox status in bacterial cells. In addition to regulating expression of respiratory enzymes, Shewanella oneidensis Arc also plays a critical role in cell envelope integrity. The absence of Arc and master envelope stress response (ESR) regulator σ^E causes a synthetic lethal phenotype. Our research shows that the Arc loss downregulates lptFG expression, leading to cell envelope defects that require σ^E-mediated ESR for viability. The complex mechanisms revealed here underscore the importance of the interplay between global regulators in bacterial adaption to their natural inhabits.

Tomato spotted wilt orthotospovirus influences the reproduction of its insect vector, western flower thrips, Frankliniella occidentalis, to facilitate transmission

BACKGROUND

Tomato spotted wilt orthotospovirus (TSWV), one of the most devastating viruses of ornamental plants and vegetable crops worldwide, is transmitted by the western flower thrips, Frankliniella occidentalis (Pergande), in a persistent-propagative manner. How TSWV influences the reproduction of its vector to enhance transmission and whether infection with TSWV changes the mating behavior of F. occidentalis are not fully understood.

RESULTS

TSWV-exposed thrips had a significantly longer developmental time than non-exposed individuals. More importantly, increased developmental time was predominantly associated with adults, a stage critical for dispersal and virus transmission. In addition, TSWV-exposed F. occidentalis produced substantially more progeny than did non-exposed thrips. Interestingly, most of the increase in progeny came from an increase in males, a sex with a greater dispersal and virus transmission capability. Specifically, the female/male ratio of progeny shifted from 1.3-7.0/1 to 0.6-1.1/1. As for mating behavior, copulation time was significantly longer in TSWV-exposed thrips. Finally, females tended to re-mate less when exposed to the virus. Resistance to re-mating may lead to reduced sperm availability in females, which translates to a larger number of male progeny under a haplodiploid system.

CONCLUSION

These combined results suggest that TSWV can influence the developmental time, mating behavior, fecundity, and offspring sex allocation of its vector F. occidentalis to facilitate virus transmission. As such, a monitoring program capable of the earlier detection of the virus in host plants and/or its insect vector, thrips, using double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), real time quantitative polymerase chain reaction (RT-qPCR) or virus detection strips might be beneficial for long-term, sustainable management. © 2020 Society of Chemical Industry.

Mutual interplay between ArcA and σE orchestrates envelope stress response in Shewanella oneidensis

To survive and thrive in harsh and ever-changing environments, intricate mechanisms have evolved for bacterial cells to monitor perturbations impacting the integrity of their envelope and to mount an appropriate response to contain or repair the damage. In this study, we report in Shewanella oneidensis a previously undescribed mechanism for the envelope defect resulting from the loss of Arc, a two-component transcriptional regulatory system crucial for respiration. We uncovered σ^E , a master regulator establishing and maintaining the integrity of the cell envelope in γ-proteobacteria, as the determining factor for the cell envelope defect of the arcA mutant. When ArcA is depleted, σ^E activity is compromised by enhanced production of anti-σ^E protein RseA. Surprisingly, S. oneidensis σ^E is not essential for viability, but becomes so in the absence of ArcA. Furthermore, we demonstrated that there is an interplay between these two regulators as arcA expression is affected by availability of σ^E . Overall, our results underscore functional interplay of regulatory systems for envelope stress response: although each of the systems may respond to perturbation of particular components of the envelope, they are functionally intertwined, working together to form an interconnected safety net.

Transcriptional regulator ArcA mediates expression of oligopeptide transport systems both directly and indirectly in Shewanella oneidensis

In γ-proteobacterial species, such as Escherichia coli, the Arc (anoxic redox control) two-component system plays a major role in mediating the metabolic transition from aerobiosis to anaerobiosis, and thus is crucial for anaerobic growth but dispensable for aerobic growth. In Shewanella oneidensis, a bacterium renowned for respiratory versatility, Arc (SoArc) primarily affects aerobic growth. To date, how this occurs has remained largely unknown although the growth defect resulting from the loss of DNA-binding response regulator SoArcA is tryptone-dependent. In this study, we demonstrated that the growth defect is in part linked to utilization of oligopeptides and di-tripeptides, and peptide uptake but not peptide degradation is significantly affected by the SoArcA loss. A systematic characterization of major small peptide uptake systems manifests that ABC peptide transporter Sap and four proton-dependent oligopeptide transporters (POTs) are responsible for transport of oligopeptides and di-tripeptides respectively. Among them, Sap and DtpA (one of POTs) are responsive to the SoarcA mutation but only dtpA is under the direct control of SoArcA. We further showed that both Sap and DtpA, when overproduced, improve growth of the SoarcA mutant. While the data firmly establish a link between transport of oligopeptides and di-tripeptides and the SoarcA mutation, other yet-unidentified factors are implicated in the growth defect resulting from the SoArcA loss.

The transcription factors ActR and SoxR differentially affect the phenazine tolerance of Agrobacterium tumefaciens

Bacteria in soils encounter redox-active compounds, such as phenazines, that can generate oxidative stress, but the mechanisms by which different species tolerate these compounds are not fully understood. Here, we identify two transcription factors, ActR and SoxR, that play contrasting yet complementary roles in the tolerance of the soil bacterium Agrobacterium tumefaciens to phenazines. We show that ActR promotes phenazine tolerance by proactively driving expression of a more energy-efficient terminal oxidase at the expense of a less efficient alternative, which may affect the rate at which phenazines abstract electrons from the electron transport chain (ETC) and thereby generate reactive oxygen species. SoxR, on the other hand, responds to phenazines by inducing expression of several efflux pumps and redox-related genes, including one of three copies of superoxide dismutase and five novel members of its regulon that could not be computationally predicted. Notably, loss of ActR is far more detrimental than loss of SoxR at low concentrations of phenazines, and also increases dependence on the otherwise functionally redundant SoxR-regulated superoxide dismutase. Our results thus raise the intriguing possibility that the composition of an organism's ETC may be the driving factor in determining sensitivity or tolerance to redox-active compounds.

The Glutaredoxin Gene, grxB, Affects Acid Tolerance, Surface Hydrophobicity, Auto-Aggregation, and Biofilm Formation in Cronobacter sakazakii

Cronobacter species are foodborne pathogens that can cause neonatal meningitis, necrotizing enterocolitis, and sepsis; they have unusual abilities to survive in environmental stresses such as acid stress. However, the factors involved in acid stress responses and biofilm formation in Cronobacter species are poorly understood. In this study, we investigated the role of grxB on cellular morphology, acid tolerance, surface hydrophobicity, auto-aggregation (AAg), motility, and biofilm formation in Cronobacter sakazakii. The deletion of grxB decreased resistance to acid stresses, and notably led to weaker surface hydrophobicity, AAg, and biofilm formation under normal and acid stress conditions, compared with those of the wild type strain; however, motility was unaffected. Therefore, grxB appears to contribute to the survival of C. sakazakii in acid stresses and biofilm formation. This is the first report to provide valuable evidence for the role of grxB in acid stress responses and biofilm formation in C. sakazakii.

A Bacterial Stress Response Regulates Respiratory Protein Complexes To Control Envelope Stress Adaptation

The Cpx envelope stress response mediates adaptation to stresses that affect protein folding within the envelope of Gram-negative bacteria. Recent transcriptome analyses revealed that the Cpx response impacts genes that affect multiple cellular functions predominantly associated with the cytoplasmic membrane. In this study, we examined the connection between the Cpx response and the respiratory complexes NADH dehydrogenase I and cytochrome bo₃ in enteropathogenic Escherichia coli We found that the Cpx response directly represses the transcription of the nuo and cyo operons and that Cpx-mediated repression of these complexes confers adaptation to stresses that compromise envelope integrity. Furthermore, we found that the activity of the aerobic electron transport chain is reduced in E. coli lacking a functional Cpx response despite no change in the transcription of either the nuo or the cyo operon. Finally, we show that expression of NADH dehydrogenase I and cytochrome bo₃ contributes to basal Cpx pathway activity and that overproduction of individual subunits can influence pathway activation. Our results demonstrate that the Cpx response gauges and adjusts the expression, and possibly the function, of inner membrane protein complexes to enable adaptation to envelope stress.IMPORTANCE Bacterial stress responses allow microbes to survive environmental transitions and conditions, such as those encountered during infection and colonization, that would otherwise kill them. Enteric microbes that inhabit or infect the gut are exposed to a plethora of stresses, including changes in pH, nutrient composition, and the presence of other bacteria and toxic compounds. Bacteria detect and adapt to many of these conditions by using envelope stress responses that measure the presence of stressors in the outermost compartment of the bacterium by monitoring its physiology. The Cpx envelope stress response plays a role in antibiotic resistance and host colonization, and we have shown that it regulates many functions at the bacterial inner membrane. In this report, we describe a novel role for the Cpx response in sensing and controlling the expression of large, multiprotein respiratory complexes at the cytoplasmic membrane of Escherichia coli The significance of our research is that it will increase our understanding of how these stress responses are involved in antibiotic resistance and the mechanisms used by bacteria to colonize the gut.

NapB in excess inhibits growth of Shewanella oneidensis by dissipating electrons of the quinol pool

Shewanella, a group of ubiquitous bacteria renowned for respiratory versatility, thrive in environments where various electron acceptors (EAs) of different chemical and physiological characteristics coexist. Despite being extensively studied, we still know surprisingly little about strategies by which multiple EAs and their interaction define ecophysiology of these bacteria. Previously, we showed that nitrite inhibits growth of the genus representative Shewanella oneidensis on fumarate and presumably some other CymA (quinol dehydrogenase)-dependent EAs by reducing cAMP production, which in turn leads to lowered expression of nitrite and fumarate reductases. In this study, we demonstrated that inhibition of fumarate growth by nitrite is also attributable to overproduction of NapB, the cytochrome c subunit of nitrate reductase. Further investigations revealed that excessive NapB per se inhibits growth on all EAs tested, including oxygen. When overproduced, NapB acts as an electron shuttle to dissipate electrons of the quinol pool, likely to extracellullar EAs, because the Mtr system, the major electron transport pathway for extracellular electron transport, is implicated. The study not only sheds light on mechanisms by which certain EAs, especially toxic ones, impact the bacterial ecophysiology, but also provides new insights into how electron shuttle c-type cytochromes regulate multi-branched respiratory networks.

Shewanella oneidensis FabB: A β-ketoacyl-ACP Synthase That Works with C16:1-ACP

It is established that Escherichia coli β-ketoacyl-ACP synthase (KAS) I (encoded by EcfabB) is the primary, if not exclusive, factor for elongation of the cis-3-decenoyl-ACP (C10:1-ACP) but not effective with C16:1- or longer-chain-ACPs. To test the extent to which these features apply to KAS I proteins in other species, in this study, we examined the physiological role of FabB in Shewanella oneidensis, an excellent model for researching type II fatty acid synthetic (FAS) system and its regulation. We showed that the loss of either FabA (the enzyme that introduces double bond) or FabB, in the absence of DesA which desaturizes C16 and C18 to generate respective C16:1 and C18:1, leads to a UFA auxotroph. However, fatty acid profiles of membrane phospholipid of the fabA and fabB mutants are significantly different, suggesting that FabB participates in steps beyond elongation of C10:1-ACP. Further analyses demonstrated that S. oneidensis FabB differs from EcFabB in that (i) it is not the only enzyme capable of catalyzing elongation of the cis-3-decenoyl-ACP produced by FabA, (ii) it plays a critical role in elongation of C16:1- and longer-chain-ACPs, and (iii) its overproduction is detrimental.

Positive regulation of the Shewanella oneidensis OmpS38, a major porin facilitating anaerobic respiration, by Crp and Fur

Major porins are among the most abundant proteins embedded in the outer membrane (OM) of Gram-negative bacteria, playing crucial roles in maintenance of membrane structural integrity and OM permeability. Although many OM proteins (especially c-type cytochromes) in Shewanella oneidensis, a research model for respiratory versatility, have been extensively studied, physiological significance of major porins remains largely unexplored. In this study, we show that OmpS38 and OmpA are two major porins, neither of which is responsive to changes in osmolarity or contributes to the intrinsic resistance to β-lactam antibiotics. However, OmpS38 but not OmpA is largely involved in respiration of non-oxygen electron acceptors. We then provide evidence that expression of ompS38 is transcribed from two promoters, the major of which is favored under anaerobic conditions while the other appears constitutive. The major promoter is under the direct control of Crp, the master regulator dictating respiration. As a result, the increase in the level of OmpS38 correlates with an elevated activity in Crp under anaerobic conditions. In addition, we show that the activity of the major promoter is also affected by Fur, presumably indirectly, the transcription factor for iron-dependent gene expression.