Structural basis for microcin C7 inactivation by the MccE acetyltransferase

S51 Family Peptidases Provide Resistance to Peptidyl-Nucleotide Antibiotic McC

Microcin C (McC)-like compounds are natural Trojan horse peptide-nucleotide antibiotics produced by diverse bacteria. The ribosomally synthesized peptide parts of these antibiotics are responsible for their facilitated transport into susceptible cells. Once inside the cell, the peptide part is degraded, releasing the toxic payload, an isoaspartyl-nucleotide that inhibits aspartyl-tRNA synthetase, an enzyme essential for protein synthesis. Bacteria that produce microcin C-like compounds have evolved multiple ways to avoid self-intoxication. Here, we describe a new strategy through the action of S51 family peptidases, which we name MccG. MccG cleaves the toxic isoaspartyl-nucleotide, rendering it inactive. While some MccG homologs are encoded by gene clusters responsible for biosynthesis of McC-like compounds, most are encoded by standalone genes whose products may provide a basal level of resistance to peptide-nucleotide antibiotics in phylogenetically distant bacteria.

Gcn5-Related N-Acetyltransferases (GNATs) With a Catalytic Serine Residue Can Play Ping-Pong Too

Enzymes in the Gcn5-related N-acetyltransferase (GNAT) superfamily are widespread and critically involved in multiple cellular processes ranging from antibiotic resistance to histone modification. While acetyl transfer is the most widely catalyzed reaction, recent studies have revealed that these enzymes are also capable of performing succinylation, condensation, decarboxylation, and methylcarbamoylation reactions. The canonical chemical mechanism attributed to GNATs is a general acid/base mechanism; however, mounting evidence has cast doubt on the applicability of this mechanism to all GNATs. This study shows that the Pseudomonas aeruginosa PA3944 enzyme uses a nucleophilic serine residue and a hybrid ping-pong mechanism for catalysis instead of a general acid/base mechanism. To simplify this enzyme's kinetic characterization, we synthesized a polymyxin B substrate analog and performed molecular docking experiments. We performed site-directed mutagenesis of key active site residues (S148 and E102) and determined the structure of the E102A mutant. We found that the serine residue is essential for catalysis toward the synthetic substrate analog and polymyxin B, but the glutamate residue is more likely important for substrate recognition or stabilization. Our results challenge the current paradigm of GNAT mechanisms and show that this common enzyme scaffold utilizes different active site residues to accomplish a diversity of catalytic reactions.

Natural Trojan horse inhibitors of aminoacyl-tRNA synthetases

For most antimicrobial compounds with intracellular targets, getting inside the cell is the major obstacle limiting their activity. To pass this barrier some antibiotics mimic the compounds of specific interest for the microbe (siderophores, peptides, carbohydrates, etc.) and hijack the transport systems involved in their active uptake followed by the release of a toxic warhead inside the cell. In this review, we summarize the information about the structures, biosynthesis, and transport of natural inhibitors of aminoacyl-tRNA synthetases (albomycin, microcin C-related compounds, and agrocin 84) that rely on such "Trojan horse" strategy to enter the cell. In addition, we provide new data on the composition and distribution of biosynthetic gene clusters reminiscent of those coding for known Trojan horse aminoacyl-tRNA synthetases inhibitors. The products of these clusters are likely new antimicrobials that warrant further investigation.

Bacteriocins to Thwart Bacterial Resistance in Gram Negative Bacteria

An overuse of antibiotics both in human and animal health and as growth promoters in farming practices has increased the prevalence of antibiotic resistance in bacteria. Antibiotic resistant and multi-resistant bacteria are now considered a major and increasing threat by national health agencies, making the need for novel strategies to fight bugs and super bugs a first priority. In particular, Gram-negative bacteria are responsible for a high proportion of nosocomial infections attributable for a large part to Enterobacteriaceae, such as pathogenic Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. To cope with their highly competitive environments, bacteria have evolved various adaptive strategies, among which the production of narrow spectrum antimicrobial peptides called bacteriocins and specifically microcins in Gram-negative bacteria. They are produced as precursor peptides that further undergo proteolytic cleavage and in many cases more or less complex posttranslational modifications, which contribute to improve their stability and efficiency. Many have a high stability in the gastrointestinal tract where they can target a single pathogen whilst only slightly perturbing the gut microbiota. Several microcins and antibiotics can bind to similar bacterial receptors and use similar pathways to cross the double-membrane of Gram-negative bacteria and reach their intracellular targets, which they also can share. Consequently, bacteria may use common mechanisms of resistance against microcins and antibiotics. This review describes both unmodified and modified microcins [lasso peptides, siderophore peptides, nucleotide peptides, linear azole(in)e-containing peptides], highlighting their potential as weapons to thwart bacterial resistance in Gram-negative pathogens and discusses the possibility of cross-resistance and co-resistance occurrence between antibiotics and microcins in Gram-negative bacteria.

Self-immunity to antibacterial peptides by ABC transporters

Bacteria produce under certain stress conditions bacteriocins and microcins that display antibacterial activity against closely related species for survival. Bacteriocins and microcins exert their antibacterial activity by either disrupting the membrane or inhibiting essential intracellular processes of the bacterial target. To this end, they can lyse bacterial membranes and cause subsequent loss of their integrity or nutrients, or hijack membrane receptors for internalisation. Both bacteriocins and microcins are ribosomally synthesised and several are posttranslationally modified, whereas others are not. Such peptides are also toxic to the producer bacteria, which utilise immunity proteins or/and dedicated ATP-binding cassette (ABC) transporters to achieve self-immunity and peptide export. In this review, we discuss the structure and mechanism of self-protection that is conferred by these ABC transporters.

Small-Molecule Acetylation by GCN5-Related N-Acetyltransferases in Bacteria

Acetylation is a conserved modification used to regulate a variety of cellular pathways, such as gene expression, protein synthesis, detoxification, and virulence. Acetyltransferase enzymes transfer an acetyl moiety, usually from acetyl coenzyme A (AcCoA), onto a target substrate, thereby modulating activity or stability. Members of the GCN5- N -acetyltransferase (GNAT) protein superfamily are found in all domains of life and are characterized by a core structural domain architecture. These enzymes can modify primary amines of small molecules or of lysyl residues of proteins. From the initial discovery of antibiotic acetylation, GNATs have been shown to modify a myriad of small-molecule substrates, including tRNAs, polyamines, cell wall components, and other toxins. This review focuses on the literature on small-molecule substrates of GNATs in bacteria, including structural examples, to understand ligand binding and catalysis. Understanding the plethora and versatility of substrates helps frame the role of acetylation within the larger context of bacterial cellular physiology.

Translation-Targeting RiPPs and Where to Find Them

Prokaryotic translation is among the major targets of diverse natural products with antibacterial activity including several classes of clinically relevant antibiotics. In this review, we summarize the information about the structure, biosynthesis, and modes of action of translation inhibiting ribosomally synthesized and post-translationally modified peptides (RiPPs). Azol(in)e-containing RiPPs are known to target translation, and several new compounds inhibiting the ribosome have been characterized recently. We performed a systematic search for biosynthetic gene clusters (BGCs) of azol(in)e-containing RiPPs. This search uncovered several groups of clusters that likely direct the synthesis of novel compounds, some of which may be targeting the ribosome.

Mechanistic and structural studies into the biosynthesis of the bacterial sugar pseudaminic acid (Pse5Ac7Ac)

The non-mammalian nonulosonic acid sugar pseudaminic acid (Pse) is present on the surface of a number of human pathogens including Campylobacter jejuni and Helicobacter pylori and other bacteria such as multidrug resistant Acinetobacter baumannii. It is likely important for evasion of the host immune sysyem, and also plays a role in bacterial motility through flagellin glycosylation. Herein we review the mechanistic and structural characterisation of the enzymes responsible for the biosynthesis of the Pse parent structure, Pse5Ac7Ac in bacteria.

Microcins in Enterobacteriaceae: Peptide Antimicrobials in the Eco-Active Intestinal Chemosphere

Microcins are low-molecular-weight, ribosomally produced, highly stable, bacterial-inhibitory molecules involved in competitive, and amensalistic interactions between Enterobacteriaceae in the intestine. These interactions take place in a highly complex chemical landscape, the intestinal eco-active chemosphere, composed of chemical substances that positively or negatively influence bacterial growth, including those originated from nutrient uptake, and those produced by the action of the human or animal host and the intestinal microbiome. The contribution of bacteria results from their effect on the host generated molecules, on food and digested food, and organic substances from microbial origin, including from bacterial degradation. Here, we comprehensively review the main chemical substances present in the human intestinal chemosphere, particularly of those having inhibitory effects on microorganisms. With this background, and focusing on Enterobacteriaceae, the most relevant human pathogens from the intestinal microbiota, the microcin’s history and classification, mechanisms of action, and mechanisms involved in microcin’s immunity (in microcin producers) and resistance (non-producers) are reviewed. Products from the chemosphere likely modulate the ecological effects of microcin activity. Several cross-resistance mechanisms are shared by microcins, colicins, bacteriophages, and some conventional antibiotics, which are expected to produce cross-effects. Double-microcin-producing strains (such as microcins MccM and MccH47) have been successfully used for decades in the control of pathogenic gut organisms. Microcins are associated with successful gut colonization, facilitating translocation and invasion, leading to bacteremia, and urinary tract infections. In fact, Escherichia coli strains from the more invasive phylogroups (e.g., B2) are frequently microcinogenic. A publicly accessible APD3 database http://aps.unmc.edu/AP/ shows particular genes encoding microcins in 34.1% of E. coli strains (mostly MccV, MccM, MccH47, and MccI47), and much less in Shigella and Salmonella (<2%). Some 4.65% of Klebsiella pneumoniae are microcinogenic (mostly with MccE492), and even less in Enterobacter or Citrobacter (mostly MccS). The high frequency and variety of microcins in some Enterobacteriaceae indicate key ecological functions, a notion supported by their dominance in the intestinal microbiota of biosynthetic gene clusters involved in the synthesis of post-translationally modified peptide microcins.

Reiterative Synthesis by the Ribosome and Recognition of the N-Terminal Formyl Group by Biosynthetic Machinery Contribute to Evolutionary Conservation of the Length of Antibiotic Microcin C Peptide Precursor

Escherichia coli microcin C (McC) is a representative member of peptide-nucleotide antibiotics produced by diverse microorganisms. The vast majority of biosynthetic gene clusters responsible for McC-like compound production encode 7-amino-acid-long precursor peptides, which are C-terminally modified by dedicated biosynthetic enzymes with a nucleotide moiety to produce a bioactive compound. In contrast, the sequences of McC-like compound precursor peptides are not conserved. Here, we studied the consequences of E. coli McC precursor peptide length increase on antibiotic production and activity. We show that increasing the precursor peptide length strongly decreases McC production by affecting multiple biosynthetic steps, suggesting that the McC biosynthesis system has evolved under significant functional constraints to maintain the precursor peptide length.

Microcin C (McC) is a peptide adenylate antibiotic produced by Escherichiacoli cells bearing a plasmid-borne mcc gene cluster. Most MccA precursors, encoded by validated mcc operons from diverse bacteria, are 7 amino acids long, but the significance of this precursor length conservation has remained unclear. Here, we created derivatives of E. colimcc operons encoding longer precursors and studied their synthesis and bioactivities. We found that increasing the precursor length to 11 amino acids and beyond strongly decreased antibiotic production. We found this decrease to depend on several parameters. First, reiterative synthesis of the MccA peptide by the ribosome was decreased at longer mccA open reading frames, leading to less efficient competition with other messenger RNAs. Second, the presence of a formyl group at the N-terminal methionine of the heptameric peptide had a strong stimulatory effect on adenylation by the MccB enzyme. No such formyl group stimulation was observed for longer peptides. Finally, the presence of the N-terminal formyl on the heptapeptide adenylate stimulated bioactivity, most likely at the uptake stage. Together, these factors should contribute to optimal activity of McC-like compounds as 7-amino-acid peptide moieties and suggest convergent evolution of several steps of the antibiotic biosynthesis pathway and their adjustment to sensitive cell uptake machinery to create a potent drug.

Comparative analysis of pyrimidine substituted aminoacyl-sulfamoyl nucleosides as potential inhibitors targeting class I aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRSs) catalyse the ATP-dependent coupling of an amino acid to its cognate tRNA. Being vital for protein translation aaRSs are considered a promising target for the development of novel antimicrobial agents. 5'-O-(N-aminoacyl)-sulfamoyl adenosine (aaSA) is a non-hydrolysable analogue of the aaRS reaction intermediate that has been shown to be a potent inhibitor of this enzyme family but is prone to chemical instability and enzymatic modification. In an attempt to improve the molecular properties of this scaffold we synthesized a series of base substituted aaSA analogues comprising cytosine, uracil and N³-methyluracil targeting leucyl-, tyrosyl- and isoleucyl-tRNA synthetases. In in vitro assays seven out of the nine inhibitors demonstrated K_i^app values in the low nanomolar range. To complement the biochemical studies, X-ray crystallographic structures of Neisseria gonorrhoeae leucyl-tRNA synthetase and Escherichia coli tyrosyl-tRNA synthetase in complex with the newly synthesized compounds were determined. These highlighted a subtle interplay between the base moiety and the target enzyme in defining relative inhibitory activity. Encouraged by this data we investigated if the pyrimidine congeners could escape a natural resistance mechanism, involving acetylation of the amine of the aminoacyl group by the bacterial N-acetyltransferases RimL and YhhY. With RimL the pyrimidine congeners were less susceptible to inactivation compared to the equivalent aaSA, whereas with YhhY the converse was true. Combined the various insights resulting from this study will pave the way for the further rational design of aaRS inhibitors.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.

Biosynthesis of the RiPP trojan horse nucleotide antibiotic microcin C is directed by the N-formyl of the peptide precursor

The N-formyl moiety of the peptide precursor directs the biosynthesis of the RiPP trojan horse nucleotide antibiotic McC.

Microcin C7 (McC) is a peptide antibiotic modified by a linkage of the terminal isoAsn amide to AMP via a phosphoramidate bond. Post-translational modification on this ribosomally produced heptapeptide precursor is carried out by MccB, which consumes two equivalents of ATP to generate the N–P linkage. We demonstrate that MccB only efficiently processes the precursor heptapeptide that retains the N-formylated initiator Met (fMet). Binding studies and kinetic measurements evidence the role of the N-formyl moiety. Structural data show that the N-formyl peptide binding results in an ordering of residues in the MccB “crossover loop”, which dictates specificity in homologous ubiquitin activating enzymes. The N-formyl peptide exhibits substrate inhibition, and cannot be displaced from MccB by the desformyl counterpart. Such substrate inhibition may be a strategy to avert unwanted McC buildup and avert toxicity in the cytoplasm of producing organisms.

Bacteriocins: perspective for the development of novel anticancer drugs

Antimicrobial peptides (AMPs) from prokaryotic source also known as bacteriocins are ribosomally synthesized by bacteria belonging to different eubacterial taxonomic branches. Most of these AMPs are low molecular weight cationic membrane active peptides that disrupt membrane by forming pores in target cell membranes resulting in cell death. While these peptides known to exhibit broad-spectrum antimicrobial activity, including antibacterial and antifungal, they displayed minimal cytotoxicity to the host cells. Their antimicrobial efficacy has been demonstrated in vivo using diverse animal infection models. Therefore, we have discussed some of the promising peptides for their ability towards potential therapeutic applications. Further, some of these bacteriocins have also been reported to exhibit significant biological activity against various types of cancer cells in different experimental studies. In fact, differential cytotoxicity towards cancer cells as compared to normal cells by certain bacteriocins directs for a much focused research to utilize these compounds as novel therapeutic agents. In this review, bacteriocins that demonstrated antitumor activity against diverse cancer cell lines have been discussed emphasizing their biochemical features, selectivity against extra targets and molecular mechanisms of action.

Comparison of Strategies to Overcome Drug Resistance: Learning from Various Kingdoms

Drug resistance, especially antibiotic resistance, is a growing threat to human health. To overcome this problem, it is significant to know precisely the mechanisms of drug resistance and/or self-resistance in various kingdoms, from bacteria through plants to animals, once more. This review compares the molecular mechanisms of the resistance against phycotoxins, toxins from marine and terrestrial animals, plants and fungi, and antibiotics. The results reveal that each kingdom possesses the characteristic features. The main mechanisms in each kingdom are transporters/efflux pumps in phycotoxins, mutation and modification of targets and sequestration in marine and terrestrial animal toxins, ABC transporters and sequestration in plant toxins, transporters in fungal toxins, and various or mixed mechanisms in antibiotics. Antibiotic producers in particular make tremendous efforts for avoiding suicide, and are more flexible and adaptable to the changes of environments. With these features in mind, potential alternative strategies to overcome these resistance problems are discussed. This paper will provide clues for solving the issues of drug resistance.

Structure and Functional Diversity of GCN5-Related N-Acetyltransferases (GNAT)

General control non-repressible 5 (GCN5)-related N-acetyltransferases (GNAT) catalyze the transfer of an acyl moiety from acyl coenzyme A (acyl-CoA) to a diverse group of substrates and are widely distributed in all domains of life. This review of the currently available data acquired on GNAT enzymes by a combination of structural, mutagenesis and kinetic methods summarizes the key similarities and differences between several distinctly different families within the GNAT superfamily, with an emphasis on the mechanistic insights obtained from the analysis of the complexes with substrates or inhibitors. It discusses the structural basis for the common acetyltransferase mechanism, outlines the factors important for the substrate recognition, and describes the mechanism of action of inhibitors of these enzymes. It is anticipated that understanding of the structural basis behind the reaction and substrate specificity of the enzymes from this superfamily can be exploited in the development of novel therapeutics to treat human diseases and combat emerging multidrug-resistant microbial infections.

Bacterial GCN5-Related N-Acetyltransferases: From Resistance to Regulation

The GCN5-related N-acetyltransferases family (GNAT) is an important family of proteins that includes more than 100000 members among eukaryotes and prokaryotes. Acetylation appears as a major regulatory post-translational modification and is as widespread as phosphorylation. N-Acetyltransferases transfer an acetyl group from acetyl-CoA to a large array of substrates, from small molecules such as aminoglycoside antibiotics to macromolecules. Acetylation of proteins can occur at two different positions, either at the amino-terminal end (αN-acetylation) or at the ε-amino group (εN-acetylation) of an internal lysine residue. GNAT members have been classified into different groups on the basis of their substrate specificity, and in spite of a very low primary sequence identity, GNAT proteins display a common and conserved fold. This Current Topic reviews the different classes of bacterial GNAT proteins, their functions, their structural characteristics, and their mechanism of action.

Crystal structure of Helicobacter pylori pseudaminic acid biosynthesis N-acetyltransferase PseH: implications for substrate specificity and catalysis

Helicobacter pylori infection is the common cause of gastroduodenal diseases linked to a higher risk of the development of gastric cancer. Persistent infection requires functional flagella that are heavily glycosylated with 5,7-diacetamido-3,5,7,9-tetradeoxy-L-glycero-L-manno-nonulosonic acid (pseudaminic acid). Pseudaminic acid biosynthesis protein H (PseH) catalyzes the third step in its biosynthetic pathway, producing UDP-2,4-diacetamido-2,4,6-trideoxy-β-L-altropyranose. It belongs to the GCN5-related N-acetyltransferase (GNAT) superfamily. The crystal structure of the PseH complex with cofactor acetyl-CoA has been determined at 2.3 Å resolution. This is the first crystal structure of the GNAT superfamily member with specificity to UDP-4-amino-4,6-dideoxy-β-L-AltNAc. PseH is a homodimer in the crystal, each subunit of which has a central twisted β-sheet flanked by five α-helices and is structurally homologous to those of other GNAT superfamily enzymes. Interestingly, PseH is more similar to the GNAT enzymes that utilize amino acid sulfamoyl adenosine or protein as a substrate than a different GNAT-superfamily bacterial nucleotide-sugar N-acetyltransferase of the known structure, WecD. Analysis of the complex of PseH with acetyl-CoA revealed the location of the cofactor-binding site between the splayed strands β4 and β5. The structure of PseH, together with the conservation of the active-site general acid among GNAT superfamily transferases, are consistent with a common catalytic mechanism for this enzyme that involves direct acetyl transfer from AcCoA without an acetylated enzyme intermediate. Based on structural homology with microcin C7 acetyltransferase MccE and WecD, the Michaelis complex can be modeled. The model suggests that the nucleotide- and 4-amino-4,6-dideoxy-β-L-AltNAc-binding pockets form extensive interactions with the substrate and are thus the most significant determinants of substrate specificity. A hydrophobic pocket accommodating the 6'-methyl group of the altrose dictates preference to the methyl over the hydroxyl group and thus to contributes to substrate specificity of PseH.

The molecular mechanism of aminopropylation of peptide-nucleotide antibiotic microcin C

Translation inhibitor microcin C (McC) is a heptapeptide with an aspartate α-carboxyl group linked to AMP via phosphoramidate bond. Modification of the McC phosphate by an aminopropyl moiety increases the biological activity by ~10-fold. Here, we determine the pathway of the aminopropylation reaction of McC. We show that the MccD enzyme uses S-adenosyl methionine to transfer 3-amino-3-carboxypropyl group onto a phosphate of an McC maturation intermediate consisting of adenylated heptapeptide. The carboxyl group is removed by the MccE enzyme, yielding mature McC. MccD is an inefficient enzyme that requires for its action the product of Escherichia coli mtn gene, a 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase, which hydrolyses 5'-methylthioadenosine, the product of MccD-catalyzed reaction, thus stimulating the amino-3-carboxypropylation reaction. Both MccD and MccE are capable of modifying McC-like compounds with divergent peptide moieties, opening way for preparation of more potent peptidyl-adenylates.

The RimL transacetylase provides resistance to translation inhibitor microcin C

Peptide-nucleotide antibiotic microcin C (McC) is produced by some Escherichia coli strains. Inside a sensitive cell, McC is processed, releasing a nonhydrolyzable analog of aspartyl-adenylate, which inhibits aspartyl-tRNA synthetase. The product of mccE, a gene from the plasmid-borne McC biosynthetic cluster, acetylates processed McC, converting it into a nontoxic compound. MccE is homologous to chromosomally encoded acetyltransferases RimI, RimJ, and RimL, which acetylate, correspondingly, the N termini of ribosomal proteins S18, S5, and L12. Here, we show that E. coli RimL, but not other Rim acetyltransferases, provides a basal level of resistance to McC and various toxic nonhydrolyzable aminoacyl adenylates. RimL acts by acetylating processed McC, which along with ribosomal protein L12 should be considered a natural RimL substrate. When overproduced, RimL also makes cells resistant to albomycin, an antibiotic that upon intracellular processing gives rise to a seryl-thioribosyl pyrimidine that targets seryl-tRNA synthetase. We further show that E. coli YhhY, a protein related to Rim acetyltransferases but without a known function, is also able to detoxify several nonhydrolyzable aminoacyl adenylates but not processed McC. We propose that RimL and YhhY protect bacteria from various toxic aminoacyl nucleotides, either exogenous or those generated inside the cell during normal metabolism.

Exploring the substrate promiscuity of an antibiotic inactivating enzyme

Peptide–nucleotide conjugates have been extensively studied as scaffolds for the development of new antibiotics.

Pseudomonas syringae self-protection from tabtoxinine-β-lactam by ligase TblF and acetylase Ttr

Plant pathogenic Pseudomonas syringae produce the hydroxy-β-lactam antimetabolite tabtoxinine-β-lactam (TβL) as a time-dependent inactivating glutamine analogue of plant glutamine synthetases. The producing pseudomonads use multiple modes of self-protection, two of which are characterized in this study. The first is the dipeptide ligase TblF which converts tabtoxinine-β-lactam to the TβL-Thr dipeptide known as tabtoxin. The dipeptide is not recognized by glutamine synthetase. This represents a Trojan Horse strategy: the dipeptide is secreted, taken up by dipeptide permeases in neighboring cells, and TβL is released by peptidase action. The second self-protection mode is elaboration by the acetyltransferase Ttr, which acetylates the α-amino group of the proximal inactivator TβL, but not the tabtoxin dipeptide.

Microcin C: biosynthesis and mechanisms of bacterial resistance

Nonhydrolyzable aminoacyl-adenylates that inhibit protein synthesis provide a promising route towards the development of novel antibiotics whose mechanism of action limits the appearance of bacterial drug resistance. The 'Trojan horse' antibiotic microcin C (McC) consists of a nonhydrolyzable aspartyl-adenylate that is efficiently imported into bacterial cells owing to a covalently attached peptide carrier. Once inside the cell, the carrier is removed by proteolytic processing to release a potent aspartyl tRNA synthetase inhibitor. The focus of this article is on the mechanism of biosynthesis of McC. We also examine the strategies utilized by McC-producing strains to overcome toxicity due to unwanted, premature processing of the drug. This article will discuss how McC biosynthesis can be systematically manipulated for the development of derivatives that will target the entire battery of aminoacyl tRNA synthetases in various bacteria.