Structural and enzymatic characterization of BacD, an L-amino acid dipeptide ligase from Bacillus subtilis

Biosynthesis of Bacillus Phosphonoalamides Reveals Highly Specific Amino Acid Ligation

Phosphonate natural products have a history of commercial success across numerous industries due to their potent inhibition of metabolic processes. Over the past decade, genome mining approaches have successfully led to the discovery of numerous bioactive phosphonates. However, continued success is dependent upon a greater understanding of phosphonate metabolism, which will enable the prioritization and prediction of biosynthetic gene clusters for targeted isolation. Here, we report the complete biosynthetic pathway for phosphonoalamides E and F, antimicrobial phosphonopeptides with a conserved C-terminal l-phosphonoalanine (PnAla) residue. These peptides, produced by Bacillus, are the direct result of PnAla biosynthesis and serial ligation by two ATP-grasp ligases. A critical step of this pathway was the reversible transamination of phosphonopyruvate to PnAla by a dedicated transaminase with preference for the forward reaction. The dipeptide ligase PnfA was shown to ligate alanine to PnAla to afford phosphonoalamide E, which was subsequently ligated to alanine by PnfB to form phosphonoalamide F. Specificity profiling of both ligases found each to be highly specific, although the limited acceptance of noncanonical substrates by PnfA allowed for in vitro formation of products incorporating alternative pharmacophores. Our findings further establish the transaminative branch of phosphonate metabolism, unveil insights into the specificity of ATP-grasp ligation, and highlight the biocatalytic potential of biosynthetic enzymes.

Effects of the dipeptides comprising leucine and lysine on lifespan and age-related stress in Caenorhabditis elegans

The aging process is affected by various stressors. An increase in oxidative stress is related to the impairment of physiological functions and enhancement of glycative stress. Food-derived bioactive peptides have various physiological functions, including antioxidant activities. Dipeptides comprising Leu and Lys (LK and KL, respectively) have been isolated from foods; however, their physiological properties remain unclear. In this study, we investigated the antioxidant/antiglycation activity of dipeptides and their antiaging effects using Caenorhabditis elegans (C. elegans). Both dipeptides showed antioxidant activities against several reactive oxygen species (ROS) in vitro. In particular, the scavenging activity of LK against superoxide radicals was higher than KL did. Moreover, dipeptides suppressed advanced glycation end products (AGEs) formation in the BSA-glucose model. In the lifespan assays using wild-type C. elegans, both LK and KL significantly prolonged the mean lifespan by 20.9% and 11.7%, respectively. In addition, LK decreased intracellular ROS and superoxide radical levels in C. elegans. Blue autofluorescence, an indicator of glycation in C. elegans with age, was also suppressed by LK. These results suggest that dipeptides, notably LK, show an antiaging effect by suppressing oxidative and glycative stress. Our findings suggest that such dipeptides can be used as a novel functional food ingredient. Food-derived dipeptide Leu-Lys (LK) and Lys-Leu (KL) exert antioxidant and antiglycation activity in vitro. Treatment with LK prolonged the mean lifespan and maximum lifespan of C. elegans more than that of KL. Intracellular ROS and blue autofluorescence levels (indicator of aging) were suppressed by LK.

Improving the enzymatic activity of l-amino acid α-ligase for imidazole dipeptide production by site-directed mutagenesis

Imidazole dipeptides, histidine-containing dipeptides, including carnosine (β-alanyl-l-histidine), anserine (β-alanyl-3-methyl-l-histidine), and balenine (β-alanyl-1-methyl-l-histidine) in animal muscles have physiological functions, such as significant antioxidant and antifatigue effects. They are obtained by extraction from natural raw materials, including chicken and fish meat. However, using natural raw materials entails stable supply and mass production limitations. l-amino acid α-ligase (Lal) catalyzes the formation of various dipeptides from unprotected l-amino acids by conjugating with adenosine 5'-triphosphate (ATP) hydrolysis reaction. In this study, site-directed mutagenesis of Lal was applied to establish an efficient method for producing imidazole dipeptides by the enzymatic process. We significantly improved the conversion rate from substrate amino acids compared with wild-type Lal.

Discovery of an ʟ-amino acid ligase implicated in Staphylococcal sulfur amino acid metabolism

Enzymes involved in Staphylococcus aureus amino acid metabolism have recently gained traction as promising targets for the development of new antibiotics, however, not all aspects of this process are understood. The ATP-grasp superfamily includes enzymes that predominantly catalyze the ATP-dependent ligation of various carboxylate and amine substrates. One subset, ʟ-amino acid ligases (LALs), primarily catalyze the formation of dipeptide products in Gram-positive bacteria, however, their involvement in S. aureus amino acid metabolism has not been investigated. Here, we present the characterization of the putative ATP-grasp enzyme (SAOUHSC_02373) from S. aureus NCTC 8325 and its identification as a novel LAL. First, we interrogated the activity of SAOUHSC_02373 against a panel of ʟ-amino acid substrates. As a result, we identified SAOUHSC_02373 as an LAL with high selectivity for ʟ-aspartate and ʟ-methionine substrates, specifically forming an ʟ-aspartyl–ʟ-methionine dipeptide. Thus, we propose that SAOUHSC_02373 be assigned as ʟ-aspartate–ʟ-methionine ligase (LdmS). To further understand this unique activity, we investigated the mechanism of LdmS by X-ray crystallography, molecular modeling, and site-directed mutagenesis. Our results suggest that LdmS shares a similar mechanism to other ATP-grasp enzymes but possesses a distinctive active site architecture that confers selectivity for the ʟ-Asp and ʟ-Met substrates. Phylogenetic analysis revealed LdmS homologs are highly conserved in Staphylococcus and closely related Gram-positive Firmicutes. Subsequent genetic analysis upstream of the ldmS operon revealed several trans-acting regulatory elements associated with control of Met and Cys metabolism. Together, these findings support a role for LdmS in Staphylococcal sulfur amino acid metabolism.

Mutagenesis of the l-Amino Acid Ligase RizA Increased the Production of Bioactive Dipeptides

The l-amino acid ligase RizA from B. subtilis selectively synthesizes dipeptides containing an N-terminal arginine. Many arginyl dipeptides have salt-taste enhancing properties while Arg-Phe has been found to have an antihypertensive effect. A total of 21 RizA variants were created by site-directed mutagenesis of eight amino acids in the substrate binding pocket. The variants were recombinantly produced in E. coli and purified by affinity chromatography. Biocatalytic reactions were set up with arginine and four amino acids differing in size and polarity (aspartic acid, serine, alanine, and phenylalanine) and were analyzed by RP-HPLC with fluorescence detection. Variant T81F significantly improved the yield in comparison to wild type RizA for aspartic acid (7 to 17%), serine (33 to 47%) and alanine (12 to 17%). S84F increased product yield similarly for aspartic acid (7 to 17%) and serine (33 to 42%). D376E increased the yield with alanine (12 to 19%) and phenylalanine (11 to 26%). The largest change was observed for S156A, which showed a yield for Arg-Phe of 40% corresponding to a 270% increase in product concentration. This study expands the knowledge about positions governing the substrate specificity of RizA and may help to inform future protein engineering endeavors.

Rational engineering of BaLal_16 from a novel Bacillus amyloliquefaciens strain to improve catalytic performance

_L-amino acid ligases (Lals) are promising biocatalysts for the synthesis of dipeptides with special biological properties. However, their poor (or broad) substrate specificity limits their industrial applications. To address this problem, a molecular engineering method for Lals was developed to enhance their catalytic performance. Based on substrate channeling, entrances to the active site for different substrates were identified, and the "gate" located around the active site pocket, which plays an essential role in substrate recognition, was then engineered to facilitate acceptance of _L-Gln. Two mutants (L110Y and N108F/L110Y) were discovered to display significantly increased catalytic activity toward _L-Ala and _L-Gln in the biosynthesis of Ala-Gln. The catalytic efficiency (k_cat/ K_m) of the L110Y and N108F/L110Y mutants was improved by 2.64-fold and 4.06-fold, respectively, compared with that of the wild type. N108F/L110Y was then further applied for batch production of Ala-Gln, which showed that the released Pi yield was 694.47 μM, which was an increase of approximately 21.4 %, and the yield of Ala-Gln was approximately 2.59 mM^-1 L^-1 mg^-1. Collectively, these findings suggest the potential practical application of this method in the rational design of Lals for increased catalytic performance.

Effective bifidogenic growth factors cyclo-Val-Leu and cyclo-Val-Ile produced by Bacillus subtilis C-3102 in the human colonic microbiota model

Bifidobacterium species are known to fulfill important functions within the human colon. Thus, stimulating the activity of bifidobacteria is important to maintain host health. We revealed that culture supernatants of Bacillus subtilis C-3102 (referred to as C-3102) stimulated the growth of Bifidobacterium species. In this study, we isolated and identified six bifidogenic growth factors, which were cyclo (D-Val-D-Ile), cyclo (L-Val-D-Ile), cyclo (D-Val-L-Ile), cyclo (L-Val-L-Ile), cyclo (D-Val-L-Leu) and cyclo (L-Val-L-Leu). These six cyclic dipeptides increased the growth of Bifidobacterium species and had no effect on potentially harmful gut organisms. Moreover, supplementation with a mixture of these six cyclic dipeptides significantly increased the abundance of microorganisms related to the genus Bifidobacterium in a human colonic microbiota model culture system, although supplementation with a single type of dipeptide had no effect. These results show that cyclic dipeptides containing Val-Leu and Val-Ile produced by C-3102 could serve as bifidogenic growth factors in the gut microbial community.

Structural insights into the catalytic mechanism of Bacillus subtilis BacF

The nonribosomal biosynthesis of the dipeptide antibiotic bacilysin is achieved by the concerted action of multiple enzymes in the Bacillus subtilis bac operon. BacF (YwfG), encoded by the bacF gene, is a fold type I pyridoxal 5-phosphate (PLP)-dependent stereospecific transaminase. Activity assays with L-phenylalanine and 4-hydroxyphenylpyruvic acid (4HPP), a chemical analogue of tetrahydrohydroxyphenylpyruvic acid (H₄HPP), revealed stereospecific substrate preferences, a finding that was consistent with previous reports on the role of this enzyme in bacilysin synthesis. The crystal structure of this dimeric enzyme was determined in its apo form as well as in substrate-bound and product-bound conformations. Two ligand-bound structures were determined by soaking BacF crystals with substrates (L-phenylalanine and 4-hydroxyphenylpyruvate). These structures reveal multiple catalytic steps: the internal aldimine with PLP and two external aldimine conformations that show the rearrangement of the external aldimine to generate product (L-tyrosine). Together, these structural snapshots provide an insight into the catalytic mechanism of this transaminase.

Biosynthesis of Oligopeptides Using ATP-Grasp Enzymes

Peptides are biologically occurring oligomers of amino acids linked by amide bonds and are indispensable for all living organisms. Many bioactive peptides are used as antibiotics, antivirus agents, insecticides, pheromones, and food preservatives. Nature employs several different strategies to form amide bonds. ATP-grasp enzymes that catalyze amide bond formation (ATP-dependent carboxylate-amine ligases) utilize a strategy of activating carboxylic acid as an acylphosphate intermediate to form amide bonds and are involved in many different biological processes in both primary and secondary metabolisms. The recent discovery of several new ATP-dependent carboxylate-amine ligases has expanded the diversity of this group of enzymes and showed their usefulness for generating oligopeptides. In this review, an overview of findings on amide bond formation catalyzed by ATP-grasp enzymes in the past decade is presented.

Effective production of Pro-Gly by mutagenesis of l-amino acid ligase

l-Amino acid ligase (Lal) catalyzes dipeptide synthesis from unprotected l-amino acids by hydrolysis ATP to ADP. Each Lal displays unique substrate specificity, and many different dipeptides can be synthesized by selecting suitable Lal. We have already successfully synthesized Met-Gly selectively by replacing the Pro85 residues of Lal from Bacillus licheniformis (BL00235). From these results, we deduced that the amino acid residue at position 85 had a key role in enzyme activity, and applied these findings to other Lals. When Pro and Gly were used as substrates, TabS from Pseudomonas syringae, synthesized the salt taste enhancing dipeptide Pro-Gly and other three dipeptides (Gly-Pro, Pro-Pro, and Gly-Gly) was hardly synthesized from its substrate specificity. However, the amount of Pro-Gly was low. Therefore, to alter the substrate specificity and increase the amount of Pro-Gly, we selected amino acid residues that might affect the enzyme activity, Ser85 corresponding to Pro85 of BL00235, and His294 on the results from previous studies and the predicted structure of TabS. These residues were replaced with 20 proteogenic amino acids, and Pro-Gly synthesizing reactions were conducted. The S85T and the H294D mutants synthesized more Pro-Gly than wild-type. Furthermore, the S85T/H294D double mutant synthesized considerably more Pro-Gly than the single mutant did. These results showed that the amino acid position 85 of TabS affect the enzyme activity similarly to BL00235. In addition, replacing the amino acid residue positioning around the N-terminal substrate and constructing the double mutant led to increase the amount of Pro-Gly.

Amides in Nature and Biocatalysis

Amides are widespread in biologically active compounds with a broad range of applications in biotechnology, agriculture and medicine. Therefore, as alternative to chemical synthesis the biocatalytic amide synthesis is a very interesting field of research. As usual, Nature can serve as guide in the quest for novel biocatalysts. Several mechanisms for carboxylate activation involving mainly acyl-adenylate, acyl-phosphate or acyl-enzyme intermediates have been discovered, but also completely different pathways to amides are found. In addition to ribosomes, selected enzymes of almost all main enzyme classes are able to synthesize amides. In this review we give an overview about amide synthesis in Nature, as well as biotechnological applications of these enzymes. Moreover, several examples of biocatalytic amide synthesis are given.

Alteration of the substrate specificity of l-amino acid ligase and selective synthesis of Met-Gly as a salt taste enhancer

Dipeptides have unique physiological functions. This study focused on the salt-taste-enhancing dipeptide Met-Gly. BL00235, an l-amino acid ligase from Bacillus licheniformis NBRC12200, synthesizes Met-Gly as a major product as well as Met-Met as a by-product. To alter the substrate specificity of BL00235 and synthesize Met-Gly selectively, we chose to alter Pro85 residue based on the BL00235 crystal structure. We predicted that Met might be not recognized as a C-terminal substrate by occupying the space around C-terminal substrate. Pro85 was replaced with Phe, Tyr, and Trp, which have bulky aromatic side chains, by site-directed mutagenesis. These mutants lost the capacity to synthesize Met-Met, during the synthesis of Met-Gly. Furthermore, they did not synthesize Met-Met, even when methionine was used as a substrate. These results show that the amino acid residue at position 85 has a key role in C-terminal substrate specificity.

Structure of RizA, an L-amino-acid ligase from Bacillus subtilis

RizA is an L-amino-acid ligase from Bacillus subtilis that participates in the biosynthesis of rhizocticin, an oligopeptide antibiotic. The substrate-free form of RizA has been crystallized and the structure was solved at 2.8 Å resolution. The amino-acid-binding site appears to be capable of accommodating multiple amino acids, consistent with previous biochemical studies.

Experimental evolution of enhanced growth by Bacillus subtilis at low atmospheric pressure: genomic changes revealed by whole-genome sequencing

Knowledge of how microorganisms respond and adapt to low-pressure (LP) environments is limited. Previously, Bacillus subtilis strain WN624 was grown at the near-inhibitory LP of 5 kPa for 1,000 generations and strain WN1106, which exhibited increased relative fitness at 5 kPa, was isolated. Genomic sequence differences between ancestral strain WN624 and LP-evolved strain WN1106 were identified using whole-genome sequencing. LP-evolved strain WN1106 carried amino acid-altering mutations in the coding sequences of only seven genes (fliI, parC, ytoI, bacD, resD, walK, and yvlD) and a single 9-nucleotide in-frame deletion in the rnjB gene that encodes RNase J2, a component of the RNA degradosome. By using a collection of frozen stocks of the LP-evolved culture taken at 50-generation intervals, it was determined that (i) the fitness increase at LP occurred rapidly, while (ii) mutation acquisition exhibited complex kinetics. A knockout mutant of rnjB was shown to increase the competitive fitness of B. subtilis at both LP and standard atmospheric pressure.

Biochemistry, genetics and regulation of bacilysin biosynthesis and its significance more than an antibiotic

Bacillus subtilis has the capacity to produce more than two dozen bioactive compounds with an amazing variety of chemical structures. Among them, bacilysin is a non-ribosomally synthesized dipeptide antibiotic consisting of l-alanine residue at the N terminus and a non-proteinogenic amino acid, l-anticapsin, at the C terminus. In spite of its simple structure, it is active against a wide range of bacteria and fungi. As a potent antimicrobial agent, we briefly review the biochemistry and genetics as well as the regulation of bacilysin biosynthesis within the frame of peptide pheromones-based control of secondary activities. Biological functions of bacilysin in the producer B. subtilis beyond its antimicrobial activity as well as potential biotechnological use of the biosynthetic enzyme l-amino acid ligase (Lal) are also discussed.

Single mutation alters the substrate specificity of L-amino acid ligase

L-Amino acid ligase (Lal) catalyzes the formation of a dipeptide from two L-amino acids in an ATP-dependent manner and belongs to the ATP-grasp superfamily. Bacillus subtilis YwfE, the first identified Lal, produces the dipeptide antibiotic bacilysin, which consists of L-Ala and L-anticapsin. Its substrate specificity is restricted to smaller amino acids such as L-Ala for the N-terminal end of the dipeptide, whereas a wide range of hydrophobic amino acids including L-Phe and L-Met are recognized for the C-terminal end in vitro. We determined the crystal structures of YwfE with bound ADP-Mg(2+)-Pi and ADP-Mg(2+)-L-Ala at 1.9 and 2.0 Å resolutions, respectively. On the basis of these structures, we generated point mutants of residues that are considered to participate in the recognition of L-Ala and measured their ATPase activity. The conserved Arg328 is suggested to be a crucial residue for L-Ala recognition and catalysis. The mutation of Trp332 to Ala caused the enzyme to hydrolyze ATP, even in the absence of l-Ala, and the structure of this mutant protein appeared to show a cavity in the N-terminal substrate-binding pocket. These results suggest that Trp332 plays a key role in restricting the substrate specificity to smaller amino acids such as L-Ala. Moreover, Trp332 mutants can alter the substrate specificity and activity depending on the size and shape of substituted amino acids. These observations provide sufficient scope for the rational design of Lal to produce desirable dipeptides. We propose that the positioning of the conserved Arg residue in Lal is important for enantioselective recognition of L-amino acids.

Structures of SAICAR synthetase (PurC) from Streptococcus pneumoniae with ADP, Mg2+, AIR and Asp

Streptococcus pneumoniae is a multidrug-resistant pathogen that is a target of considerable interest for antibacterial drug development. One strategy for drug discovery is to inhibit an essential metabolic enzyme. The seventh step of the de novo purine-biosynthesis pathway converts carboxyaminoimidazoleribonucleotide (CAIR) and L-aspartic acid (Asp) to 4-(N-succino)-5-aminoimidazole-4-carboxamide ribonucleotide (SAICAR) in the presence of adenosine 5'-triphosphate (ATP) using the enzyme PurC. PurC has been shown to be conditionally essential for bacterial replication. Two crystal structures of this essential enzyme from Streptococcus pneumoniae (spPurC) in the presence of adenosine 5'-diphosphate (ADP), Mg(2+), aminoimidazoleribonucleotide (AIR) and/or Asp have been obtained. This is the first structural study of spPurC, as well as the first of PurC from any species with Asp in the active site. Based on these findings, two model structures are proposed for the active site with all of the essential ligands (ATP, Mg(2+), Asp and CAIR) present, and a relay mechanism for the formation of the product SAICAR is suggested.

L-amino acid ligase from Pseudomonas syringae producing tabtoxin can be used for enzymatic synthesis of various functional peptides

Functional peptides are expected to be beneficial compounds that improve our quality of life. To address the growing need for functional peptides, we have examined peptide synthesis by using microbial enzymes. l-Amino acid ligase (Lal) catalyzes the condensation of unprotected amino acids in an ATP-dependent manner and is applicable to fermentative production. Hence, Lal is a promising enzyme to achieve cost-effective synthesis. To obtain a Lal with novel substrate specificity, we focused on the putative Lal involved in the biosynthesis of the dipeptidic phytotoxin designated tabtoxin. The tabS gene was cloned from Pseudomonas syringae NBRC14081 and overexpressed in Escherichia coli cells. The recombinant TabS protein produced showed the broadest substrate specificity of any known Lal; it detected 136 of 231 combinations of amino acid substrates when dipeptide synthesis was examined. In addition, some new substrate specificities were identified and unusual amino acids, e.g., l-pipecolic acid, hydroxy-l-proline, and β-alanine, were found to be acceptable substrates. Furthermore, kinetic analysis and monitoring of the reactions over a short time revealed that TabS showed distinct substrate selectivity at the N and C termini, which made it possible to specifically synthesize a peptide without by-products such as homopeptides and heteropeptides with the reverse sequence. TabS specifically synthesized the following functional peptides, including their precursors: l-arginyl-l-phenylalanine (antihypertensive effect; yield, 62%), l-leucyl-l-isoleucine (antidepressive effect; yield, 77%), l-glutaminyl-l-tryptophan (precursor of l-glutamyl-l-tryptophan, which has antiangiogenic activity; yield, 54%), l-leucyl-l-serine (enhances saltiness; yield, 83%), and l-glutaminyl-l-threonine (precursor of l-glutamyl-l-threonine, which enhances saltiness; yield, 96%). Furthermore, our results also provide new insights into tabtoxin biosynthesis.

Action and timing of BacC and BacD in the late stages of biosynthesis of the dipeptide antibiotic bacilysin

Biosynthesis of the dipeptide antibiotic bacilysin, encoded by the seven Bacillus subtilis genes bacA-G, involves diversion of flux from prephenate to the noncognate amino acid anticapsin. The anticapsin warhead is then ligated to the C-terminus of l-alanine to produce mature bacilysin. We have previously noted the formation of two diastereomers of tetrahydrotyrosine (4S- and 4R-H(4)Tyr) by tandem action of the four purified enzymes BacABGF. BacC (oxidase) and BacD (ligase) have been hypothesized to be remaining late stage enzymes in bacilysin biosynthesis. Using a combination of BacCD in vitro studies, B. subtilis deletion mutants, and isotopic feeding studies, we were able to determine that the H(4)Tyr diastereomers are actually shunt products that are not on-pathway to bacilysin biosynthesis. Dihydroanticapsin and dihydrobacilysin accumulate in extracts of a ΔbacC strain and are processed to anticapsin and then bacilysin upon addition of BacC and BacD, respectively. These results suggest the epoxide group in bacilysin is installed in an earlier step of bacilysin biosynthesis, while BacC oxidation of the C(7)-hydroxyl and the subsequent BacD ligation of anticapsin to l-Ala are the penultimate and ultimate steps of bacilysin biosynthesis, respectively.

Synthesis and modifications of phosphinic dipeptide analogues

Pseudopeptides containing the phosphinate moiety (-P(O)(OH)CH₂-) have been studied extensively, mainly as transition state analogue inhibitors of metalloproteases. The key synthetic aspect of their chemistry is construction of phosphinic dipeptide derivatives bearing appropriate side-chain substituents. Typically, this synthesis involves a multistep preparation of two individual building blocks, which are combined in the final step. As this methodology does not allow simple variation of the side-chain structure, many efforts have been dedicated to the development of alternative approaches. Recent achievements in this field are summarized in this review. Improved methods for the formation of the phosphinic peptide backbone, including stereoselective and multicomponent reactions, are presented. Parallel modifications leading to the structurally diversified substituents are also described. Finally, selected examples of the biomedical applications of the title compounds are given.

The structure of L-amino-acid ligase from Bacillus licheniformis

L-Amino-acid ligases (LALs) are enzymes which catalyze the formation of dipeptides by linking two L-amino acids. Although many dipeptides are known and expected to have medical and nutritional benefits, their practical use has been limited owing to their low availability and high expense. LALs are potentially desirable tools for the efficient production of dipeptides; however, the molecular basis of substrate recognition by LAL has not yet been sufficiently elucidated for the design of ideal LALs for the desired dipeptides. This report presents the crystal structure of the LAL BL00235 derived from Bacillus licheniformis NBRC 12200 determined at 1.9 Å resolution using the multi-wavelength anomalous dispersion method. The overall structure of BL00235 is fairly similar to that of YwfE, the only LAL with a known structure, but the structure around the catalytic site contains some significant differences. Detailed structural comparison of BL00235 with YwfE sheds some light on the molecular basis of the substrate specificities.

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.