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Aminoacyl tRNA synthetases are enzymes involved in the key process of coupling an amino acid to its cognate tRNA. AN3365 is a novel antibiotic that specifically targets leucyl-tRNA synthetase, whose development was halted after evaluation in phase II clinical trials owing to the rapid selection of resistance. In an attempt to bring AN3365 back into the developmental pipeline we have evaluated the efficacy of AN3365 in combination with different classes of antibiotic and characterized its mechanism of action. Although we detect no synergy or antagonism in combination with a range of antibiotic classes, a combination of AN3365 with colistin reduces the accumulation of AN3365-resistant and colistin resistance mutations. We also demonstrate that treatment with AN3365 results in the dramatic accumulation of the alarmone (p)ppGpp, the effector of the stringent response-a key player in antibiotic tolerance., Competing Interests: Compliance with ethical standards Ethical approval was not required. Funding This work was supported by a grant from the European Union Framework 7 (FP7) programme—Health.2013.2.31-1- NABARSI (grant agreement no: 601725—www.nabarsi.eu). Conflicts of interest The authors declare that they have no conflicts of interest.

Objectives: Carbapenemase-resistant bacteria are increasingly spreading worldwide causing public concern due to their ability to elude antimicrobial treatment. Early identification of these bacteria is therefore of high importance. Here, we describe the development of a simple and robust protocol for the detection of carbapenemase activity in clinical isolates of Enterobacteriaceae, suitable for routine and clinical applications., Methods: The final protocol involves cellular lysis and enzyme extraction from a defined amount of bacterial cells followed by the addition of a benchmark drug (e.g. the carbapenem antibiotic imipenem or ertapenem). Carbapenem inactivation is mediated by enzymatic hydrolysis (cleavage) of the β-lactam common structural motif, which can be detected using MALDI-TOF MS., Results: A total of 260 strains were studied (208 carbapenemase producers and 52 non-carbapenemase producers) resulting in 100% sensitivity and 100% specificity for the KPC, NDM and OXA-48-like PCR-confirmed positive isolates using imipenem as benchmark. Differences between the benchmark (indicator) antibiotics imipenem and ertapenem, buffer constituents and sample preparation methods have been investigated. Carbapenemase activity was further characterized by performing specific inhibitor experiments. Intraday and interday reproducibility (coefficient of variation) of the observed hydrolysis results were 15% and 30%, respectively. A comparative study of our extraction method and a recently published method using whole bacterial cells is presented and differences are discussed., Conclusions: Using this method, an existing carbapenemase activity can be directly read from the mass spectrum as a ratio of hydrolysed product and substrate, setting an important step towards routine application in clinical laboratories., (© The Author 2016. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.)

Background: Bacillus subtilis is an important cell factory for the biotechnological industry due to its ability to secrete commercially relevant proteins in large amounts directly into the growth medium. However, hyper-secretion of proteins, such as α-amylases, leads to induction of the secretion stress-responsive CssR-CssS regulatory system, resulting in up-regulation of the HtrA and HtrB proteases. These proteases degrade misfolded proteins secreted via the Sec pathway, resulting in a loss of product. The aim of this study was to investigate the secretion stress response in B. subtilis 168 cells overproducing the industrially relevant α-amylase AmyM from Geobacillus stearothermophilus, which was expressed from the strong promoter P(amyQ)-M., Results: Here we show that activity of the htrB promoter as induced by overproduction of AmyM was "noisy", which is indicative for heterogeneous activation of the secretion stress pathway. Plasmids were constructed to allow real-time analysis of P(amyQ)-M promoter activity and AmyM production by, respectively, transcriptional and out-of-frame translationally coupled fusions with gfpmut3. Our results show the emergence of distinct sub-populations of high- and low-level AmyM-producing cells, reflecting heterogeneity in the activity of P(amyQ)-M. This most likely explains the heterogeneous secretion stress response. Importantly, more homogenous cell populations with regard to P(amyQ)-M activity were observed for the B. subtilis mutant strain 168degUhy32, and the wild-type strain 168 under optimized growth conditions., Conclusion: Expression heterogeneity of secretory proteins in B. subtilis can be suppressed by degU mutation and optimized growth conditions. Further, the out-of-frame translational fusion of a gene for a secreted target protein and gfp represents a versatile tool for real-time monitoring of protein production and opens novel avenues for Bacillus production strain improvement.

The Tat system transports folded proteins across the bacterial plasma membrane. The mechanism is believed to involve coalescence of a TatC-containing unit with a separate TatA complex, but the full translocation complex has never been visualised and the assembly process is poorly defined. We report the analysis of the Bacillus subtilis TatAyCy system, which occurs as separate TatAyCy and TatAy complexes at steady state, using single-particle electron microscopy (EM) and advanced atomic force microscopy (AFM) approaches. We show that a P2A mutation in the TatAy subunit leads to apparent super-assembly of Tat complexes. Purification of TatCy-containing complexes leads to a large increase in the TatA:TatC ratio, suggesting that TatAy(P2A) complexes may have attached to the TatAyCy complex. EM and AFM analyses show that the wild-type TatAyCy complex purifies as roughly spherical complexes of 9-16nm diameter, whereas the P2A mutation leads to accumulation of large (up to 500nm long) fibrils that are chains of numerous complexes. Time lapsed AFM imaging, recorded on fibrils under liquid, shows that they adopt a variety of tightly curved conformations, with radii of curvature of 10-12nm comparable to the size of single TatAy(P2A) complexes. The combined data indicate that the mutation leads to super-assembly of TatAy(P2A) complexes and we propose that an individual TatAy(P2A) complex assembles initially with a TatAy(P2A)Cy complex, after which further TatAy(P2A) complexes attach to each other. The data further suggest that the N-terminal extracytoplasmic domain of TatAy plays an essential role in Tat complex interactions., (Copyright © 2014. Published by Elsevier B.V.)

The twin-arginine protein translocation (Tat) system has a unique ability to translocate folded and co-factor-containing proteins across lipid bilayers. The Tat pathway is present in bacteria, archaea and in the thylakoid membranes of chloroplasts and, depending on the organism and environmental conditions, it can be deemed important for cell survival, virulence or bioproduction. This review provides an overview of the current understanding of the Tat system with specific focus on Gram-positive bacteria. The 'universal minimal Tat system' is composed of a TatA and a TatC protein. However, this pathway is more commonly composed of two TatA-like proteins and one TatC protein. Often the TatA-like proteins have diverged to have two different functions and, in this case, the second TatA-like protein is usually referred to as TatB. The correct folding and/or incorporation of co-factors are requirements for translocation, and the known quality control mechanisms are examined in this review. A number of examples of crosstalk between the Tat system and other protein transport systems, such as the Sec-YidC translocon and signal peptidases or sheddases are also discussed. Further, an overview of specific Gram-positive bacterial Tat systems found in monoderm and diderm species is detailed. Altogether, this review highlights the unique features of Gram-positive bacterial Tat systems and pinpoints key questions that remain to be addressed in future research. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey., (© 2013.)

The twin-arginine translocation (Tat) pathway can transport folded and co-factor-containing cargo proteins over bacterial cytoplasmic membranes. Functional Tat machinery components, a folded state of the cargo protein and correct co-factor insertion in the cargo protein are generally considered as prerequisites for successful translocation. The present studies were aimed at a dissection of these requirements with regard to the Rieske iron-sulfur protein QcrA of Bacillus subtilis. Notably, QcrA is a component of the cytochrome bc1 complex, which is conserved from bacteria to man. Single amino acid substitutions were introduced into the Rieske domain of QcrA to prevent either co-factor binding or disulfide bond formation. Both types of mutations precluded QcrA translocation. Importantly, a proofreading hierarchy was uncovered, where a QcrA mutant defective in disulfide bonding was quickly degraded, whereas mutant QcrA proteins defective in co-factor binding accumulated in the cytoplasm and membrane. Altogether, these are the first studies on Tat-dependent protein translocation where both oxidative folding and co-factor attachment have been addressed in a single native molecule.

The general protein secretion pathway of Bacillus subtilis has a high capacity for protein export from the cytoplasm, which is exploited in the biotechnological production of a wide range of enzymes. These exported proteins pass the membrane in an unfolded state, and accordingly, they have to fold into their active and protease-resistant conformations once membrane passage is completed. The lipoprotein PrsA and the membrane proteins HtrA and HtrB facilitate the extracytoplasmic folding and quality control of exported proteins. Among the native exported proteins of B. subtilis are at least 10 proteases that have previously been implicated in the degradation of heterologous secreted proteins. Recently, we have shown that these proteases also degrade many native membrane proteins, lipoproteins, and secreted proteins. The present studies were therefore aimed at assessing to what extent these proteases also degrade extracytoplasmic catalysts for protein folding. To this end, we employed a collection of markerless protease mutant strains that lack up to 10 different extracytoplasmic proteases. The results show that PrsA, HtrA, and HtrB are indeed substrates of multiple extracytoplasmic proteases. Thus, improved protein secretion by multiple-protease-mutant strains may be related to both reduced proteolysis and improved posttranslocational protein folding and quality control.

Efficient uptake of iron is of critical importance for growth and viability of microbial cells. Nevertheless, several mechanisms for iron uptake are not yet clearly defined. Here we report that the widely conserved transporter EfeUOB employs an unprecedented dual-mode mechanism for acquisition of ferrous (Fe[II]) and ferric (Fe[III]) iron in the bacterium Bacillus subtilis. We show that the binding protein EfeO and the permease EfeU form a minimal complex for ferric iron uptake. The third component EfeB is a hemoprotein that oxidizes ferrous iron to ferric iron for uptake by EfeUO. Accordingly, EfeB promotes growth under microaerobic conditions where ferrous iron is more abundant. Notably, EfeB also fulfills a vital role in cell envelope stress protection by eliminating reactive oxygen species that accumulate in the presence of ferrous iron. In conclusion, the EfeUOB system contributes to the high-affinity uptake of iron that is available in two different oxidation states., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Tat-dependent protein transport permits the traffic of fully folded proteins across membranes in bacteria and chloroplasts. The mechanism by which this occurs is not understood. Current theories propose that a key step requires the coalescence of a substrate-binding TatC-containing complex with a TatA complex, which forms pores of varying sizes that could accommodate different substrates. We have studied the structure of the TatAd complex from Bacillus subtilis using electron microscopy to generate the first 3D model of a TatA complex from a Gram-positive bacterium. We observe that TatAd does not exhibit the remarkable heterogeneity of Escherichia coli TatA complexes but instead forms ring-shaped complexes of 7.5-9nm diameter with potential pores of 2.5-3nm diameter that are occluded at one end. Such structures are consistent with those seen for E. coli TatE complexes. Furthermore, the small diameter of the TatAd pore, and the homogeneous nature of the complexes, suggest that TatAd cannot form the translocation channel by itself. Biochemical data indicate that another B. subtilis TatA complex, TatAc, has similar properties, suggesting a common theme for TatA-type complexes from Bacillus., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Bacteria employ twin-arginine translocation (Tat) pathways for the transport of folded proteins to extracytoplasmic destinations. In recent years, most studies on bacterial Tat pathways addressed the membrane-bound TatA(B)C subunits of the Tat translocase, and the specific interactions between this translocase and its substrate proteins. In contrast, relatively few studies investigated possible coactors in the TatA(B)C-dependent protein translocation process. The present studies were aimed at identifying interaction partners of the Tat pathway of Bacillus subtilis, which is a paradigm for studies on protein secretion by Gram-positive bacteria. Specifically, 36 interaction partners of the TatA and TatC subunits were identified by rigorous application of the yeast two-hybrid (Y2H) approach. Our Y2H analyses revealed that the three TatA isoforms of B. subtilis can form homo- and heterodimers. Subsequently, the secretion of the Tat substrates YwbN and PhoD was tested in mutant strains lacking genes for the TatAC interaction partners identified in our genome-wide Y2H screens. Our results show that the cell wall-bound protease WprA is important for YwbN secretion, and that the HemAT and CsbC proteins are required for PhoD secretion under phosphate starvation conditions. Taken together, our findings imply that the Bacillus Tat pathway is embedded in an intricate protein-protein interaction network., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Bacterial twin-arginine translocases can export fully folded proteins from the cytoplasm. Such proteins are usually resistant to proteolysis. Here we show that multiple extracellular proteases degrade the B. subtilis Tat substrate YwbN. This suggests either that secreted YwbN is not fully folded or that folded YwbN exposes protease cleavage sites.

The Gram-positive bacterium Bacillus subtilis contains two Tat translocases, which can facilitate transport of folded proteins across the plasma membrane. Previous research has shown that Tat-dependent protein secretion in B. subtilis is a highly selective process and that heterologous proteins, such as the green fluorescent protein (GFP), are poor Tat substrates in this organism. Nevertheless, when expressed in Escherichia coli, both B. subtilis Tat translocases facilitated exclusively Tat-dependent export of folded GFP when the twin-arginine (RR) signal peptides of the E. coli AmiA, DmsA, or MdoD proteins were attached. Therefore, the present studies were aimed at determining whether the same RR signal peptide-GFP precursors would also be exported Tat dependently in B. subtilis. In addition, we investigated the secretion of GFP fused to the full-length YwbN protein, a strict Tat substrate in B. subtilis. Several investigated GFP fusion proteins were indeed secreted in B. subtilis, but this secretion was shown to be completely Tat independent. At high-salinity growth conditions, the Tat-independent secretion of GFP as directed by the RR signal peptides from the E. coli AmiA, DmsA, or MdoD proteins was significantly enhanced, and this effect was strongest in strains lacking the TatAy-TatCy translocase. This implies that high environmental salinity has a negative influence on the avoidance of Tat-independent secretion of AmiA-GFP, DmsA-GFP, and MdoD-GFP. We conclude that as-yet-unidentified control mechanisms reject the investigated GFP fusion proteins for translocation by the B. subtilis Tat machinery and, at the same time, set limits to their Tat-independent secretion, presumably via the Sec pathway.

The twin-arginine translocation (Tat) pathway is dedicated to the transport of fully folded proteins across the cytoplasmic membranes of many bacteria and the chloroplast thylakoidal membrane. Accordingly, Tat-dependently translocated proteins are known to be delivered to the periplasm of Gram-negative bacteria, the growth medium of Gram-positive bacteria, and the thylakoid lumen. Here, we present the first example of a protein, YkuE of Bacillus subtilis, that is specifically targeted by the Tat pathway to the cell wall of a Gram-positive bacterium. The cell wall binding of YkuE is facilitated by electrostatic interactions. Interestingly, under particular conditions, YkuE can also be targeted to the cell wall in a Tat-independent manner. The biological function of YkuE was so far unknown. Our present studies show that YkuE is a metal-dependent phosphoesterase that preferentially binds manganese and zinc.

Two independent twin-arginine translocases (Tat) for protein secretion were previously identified in the Gram-positive bacterium Bacillus subtilis. These consist of the TatAd-TatCd and TatAy-TatCy subunits. The function of a third TatA subunit named TatAc was unknown. Here, we show that TatAc can form active protein translocases with TatCd and TatCy.

Twin-arginine protein translocation (Tat) pathways are required for transport of folded proteins across bacterial, archaeal and chloroplast membranes. Recent studies indicate that Tat has evolved into a mainstream pathway for protein secretion in certain halophilic archaea, which thrive in highly saline environments. Here, we investigated the effects of environmental salinity on Tat-dependent protein secretion by the Gram-positive soil bacterium Bacillus subtilis, which encounters widely differing salt concentrations in its natural habitats. The results show that environmental salinity determines the specificity and need for Tat-dependent secretion of the Dyp-type peroxidase YwbN in B. subtilis. Under high salinity growth conditions, at least three Tat translocase subunits, namely TatAd, TatAy and TatCy, are involved in the secretion of YwbN. Yet, a significant level of Tat-independent YwbN secretion is also observed under these conditions. When B. subtilis is grown in medium with 1% NaCl or without NaCl, the secretion of YwbN depends strictly on the previously described "minimal Tat translocase" consisting of the TatAy and TatCy subunits. Notably, in medium without NaCl, both tatAyCy and ywbN mutants display significantly reduced exponential growth rates and severe cell lysis. This is due to a critical role of secreted YwbN in the acquisition of iron under these conditions. Taken together, our findings show that environmental conditions, such as salinity, can determine the specificity and need for the secretion of a bacterial Tat substrate.