Regulation of the type I protein secretion system by the MisR/MisS two-component system in Neisseria meningitidis

Both MisR (CpxR) and MisS (CpxA) Are Required for Neisseria gonorrhoeae Infection in a Murine Model of Lower Genital Tract Infection

During infection, Neisseria gonorrhoeae senses and responds to stress; such responses may be modulated by MisRS (NGO0177 and NGO0176), a two-component system that is a homolog of CpxRA. In Escherichia coli, CpxRA senses and responds to envelope stress; CpxA is a sensor kinase/phosphatase for CpxR, a response regulator. When a cpxA mutant is grown in medium containing glucose, CpxR is phosphorylated by acetyl phosphate but cannot be dephosphorylated, resulting in constitutive activation. Kandler and coworkers (J. L. Kandler, C. L. Holley, J. L. Reimche, V. Dhulipala, J. T. Balthazar, A. Muszyński, R. W. Carlson, and W. M. Shafer, Antimicrob Agents Chemother 60:4690-4700, 2016, https://doi.org/10.1128/AAC.00823-16) showed that MisR (CpxR) is required for the maintenance of membrane integrity and resistance to antimicrobial peptides, suggesting a role in gonococcal survival in vivo Here, we evaluated the contributions of MisR and MisS (CpxA) to gonococcal infection in a murine model of cervicovaginal colonization and identified MisR-regulated genes using RNA sequencing (RNA-Seq). The deletion of misR or misS severely reduced the capacity of N. gonorrhoeae to colonize mice or maintain infection over a 7-day period and reduced microbial fitness after exposure to heat shock. Compared to the wild type (WT), the inactivation of misR identified 157 differentially regulated genes, most of which encoded putative envelope proteins. The inactivation of misS identified 17 differentially regulated genes compared to the WT and 139 differentially regulated genes compared to the misR mutant, 111 of which overlapped those differentially expressed in the comparison of the WT versus the misR mutant. These data indicate that an intact MisRS system is required for gonococcal infection of mice. Provided the MisR is constitutively phosphorylated in the misS mutant, the data suggest that controlled but not constitutive activation is required for gonococcal infection in mice.

A sequence-based two-level method for the prediction of type I secreted RTX proteins

Many Gram-negative bacteria use the type I secretion system (T1SS) to translocate a wide range of substrates (type I secreted RTX proteins, T1SRPs) from the cytoplasm across the inner and outer membrane in one step to the extracellular space. Since T1SRPs play an important role in pathogen-host interactions, identifying them is crucial for a full understanding of the pathogenic mechanism of T1SS. However, experimental identification is often time-consuming and expensive. In the post-genomic era, it becomes imperative to predict new T1SRPs using information from the amino acid sequence alone when new proteins are being identified in a high-throughput mode. In this study, we report a two-level method for the first attempt to identify T1SRPs using sequence-derived features and the random forest (RF) algorithm. At the full-length sequence level, the results show that the unique feature of T1SRPs is the presence of variable numbers of the calcium-binding RTX repeats. These RTX repeats have a strong predictive power and so T1SRPs can be well distinguished from non-T1SRPs. At another level, different from that of the secretion signal, we find that a sequence segment located at the last 20-30 C-terminal amino acids may contain important signal information for T1SRP secretion because obvious differences were shown between the corresponding positions of T1SRPs and non-T1SRPs in terms of amino acid and secondary structure compositions. Using five-fold cross-validation, overall accuracies of 97% at the full-length sequence level and 89% at the secretion signal level were achieved through feature evaluation and optimization. Benchmarking on an independent dataset, our method could correctly predict 63 and 66 of 74 T1SRPs at the full-length sequence and secretion signal levels, respectively. We believe that this study will be useful in elucidating the secretion mechanism of T1SS and facilitating hypothesis-driven experimental design and validation.

The Type 1 secretion pathway - the hemolysin system and beyond

Type 1 secretion systems (T1SS) are wide-spread among Gram-negative bacteria. An important example is the secretion of the hemolytic toxin HlyA from uropathogenic strains. Secretion is achieved in a single step directly from the cytosol to the extracellular space. The translocation machinery is composed of three indispensable membrane proteins, two in the inner membrane, and the third in the outer membrane. The inner membrane proteins belong to the ABC transporter and membrane fusion protein families (MFPs), respectively, while the outer membrane component is a porin-like protein. Assembly of the three proteins is triggered by accumulation of the transport substrate (HlyA) in the cytoplasm, to form a continuous channel from the inner membrane, bridging the periplasm and finally to the exterior. Interestingly, the majority of substrates of T1SS contain all the information necessary for targeting the polypeptide to the translocation channel - a specific sequence at the extreme C-terminus. Here, we summarize our current knowledge of regulation, channel assembly, translocation of substrates, and in the case of the HlyA toxin, its interaction with host membranes. We try to provide a complete picture of structure function of the components of the translocation channel and their interaction with the substrate. Although we will place the emphasis on the paradigm of Type 1 secretion systems, the hemolysin A secretion machinery from E. coli, we also cover as completely as possible current knowledge of other examples of these fascinating translocation systems. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

The Neisseria meningitidis ZnuD zinc receptor contributes to interactions with epithelial cells and supports heme utilization when expressed in Escherichia coli

Neisseria meningitidis employs redundant heme acquisition mechanisms, including TonB receptor-dependent and receptor-independent uptakes. The TonB-dependent zinc receptor ZnuD shares significant sequence similarity to HumA, a heme receptor of Moraxella catarrhalis, and contains conserved motifs found in many heme utilization proteins. We present data showing that, when expressed in Escherichia coli, ZnuD allowed heme capture on the cell surface and supported the heme-dependent growth of an E. coli hemA strain. Heme agarose captured ZnuD in enriched outer membrane fractions, and this binding was inhibited by excess free heme, supporting ZnuD's specific interaction with heme. However, no heme utilization defect was detected in the meningococcal znuD mutant, likely due to unknown redundant TonB-independent heme uptake mechanisms. Meningococcal replication within epithelial cells requires a functional TonB, and we found that both the znuD and tonB mutants were defective not only in survival within epithelial cells but also in adherence to and invasion of epithelial cells. Ectopic complementation rescued these phenotypes. Interestingly, while znuD expression was repressed by Zur with zinc as a cofactor, it also was induced by iron in a Zur-independent manner. A specific interaction of meningococcal Fur protein with the znuD promoter was demonstrated by electrophoretic mobility shift assay (EMSA). Thus, the meningococcal ZnuD receptor likely participates in both zinc and heme acquisition, is regulated by both Zur and Fur, and is important for meningococcal interaction with epithelial cells.

Characterization of DsbD in Neisseria meningitidis

Proper periplasmic disulfide bond formation is important for folding and stability of many secreted and membrane proteins, and is catalysed by three DsbA oxidoreductases in Neisseria meningitidis. DsbD provides reducing power to DsbC that shuffles incorrect disulfide bond in misfolded proteins as well as to the periplasmic enzymes that reduce apo-cytochrome c (CcsX) or repair oxidative protein damages (MrsAB). The expression of dsbD, but not other dsb genes, is positively regulated by the MisR/S two-component system. Quantitative real-time PCR analyses showed significantly reduced dsbD expression in all misR/S mutants, which was rescued by genetic complementation. The direct and specific interaction of MisR with the upstream region of the dsbD promoter was demonstrated by electrophoretic mobility shift assay, and the MisR binding sequences were mapped. Further, the expression of dsbD was found to be induced by dithiothrietol (DTT), through the MisR/S regulatory system. Surprisingly, we revealed that inactivation of dsbD can only be achieved in a strain carrying an ectopically located dsbD, in the dsbA1A2 double mutant or in the dsbA1A2A3 triple mutant, thus DsbD is indispensable for DsbA-catalysed oxidative protein folding in N. meningitidis. The defects of the meningococcal dsbA1A2 mutant in transformation and resistance to oxidative stress were more severe in the absence of dsbD.

Regulatory role of the MisR/S two-component system in hemoglobin utilization in Neisseria meningitidis

Outer membrane iron receptors are some of the major surface entities that are critical for meningococcal pathogenesis. The gene encoding the meningococcal hemoglobin receptor, HmbR, is both independently transcribed and transcriptionally linked to the upstream gene hemO, which encodes a heme oxygenase. The MisR/S two-component system was previously determined to regulate hmbR transcription, and its hemO and hmbR regulatory mechanisms were characterized further here. The expression of hemO and hmbR was downregulated in misR/S mutants under both iron-replete and iron-restricted conditions, and the downregulation could be reversed by complementation. No significant changes in expression of other iron receptors were detected, suggesting that the MisR/S system specifically regulates hmbR. When hemoglobin was the sole iron source, growth defects were detected in the mutants. Primer extension analysis identified a promoter upstream of the hemO-associated Correia element (CE) and another promoter at the proximal end of CE, and processed transcripts previously identified for other cotranscribed CEs were also detected, suggesting that there may be posttranscriptional regulation. MisR directly interacts with sequences upstream of the CE and upstream of the hmbR Fur binding site and thus independently regulates hemO and hmbR. Analysis of transcriptional reporters of hemO and hmbR further demonstrated the positive role of the MisR/S system and showed that the transcription of hmbR initiated from hemO was significantly reduced. A comparison of the effects of the misS mutation under iron-replete and iron-depleted conditions suggested that activation by the MisR/S system and iron-mediated repression by Fur act independently. Thus, the expression of hemO and hmbR is coordinately controlled by multiple independent regulatory mechanisms, including the MisR/S two-component system.