Posttranslational Backbone Modifications in the Ribosomal Biosynthesis of the Glycine-Rich Antibiotic Microcin B17

Friend or Foe: Protein Inhibitors of DNA Gyrase

DNA gyrase is essential for the successful replication of circular chromosomes, such as those found in most bacterial species, by relieving topological stressors associated with unwinding the double-stranded genetic material. This critical central role makes gyrase a valued target for antibacterial approaches, as exemplified by the highly successful fluoroquinolone class of antibiotics. It is reasonable that the activity of gyrase could be intrinsically regulated within cells, thereby helping to coordinate DNA replication with doubling times. Numerous proteins have been identified to exert inhibitory effects on DNA gyrase, although at lower doses, it can appear readily reversible and therefore may have regulatory value. Some of these, such as the small protein toxins found in plasmid-borne addiction modules, can promote cell death by inducing damage to DNA, resulting in an analogous outcome as quinolone antibiotics. Others, however, appear to transiently impact gyrase in a readily reversible and non-damaging mechanism, such as the plasmid-derived Qnr family of DNA-mimetic proteins. The current review examines the origins and known activities of protein inhibitors of gyrase and highlights opportunities to further exert control over bacterial growth by targeting this validated antibacterial target with novel molecular mechanisms. Furthermore, we are gaining new insights into fundamental regulatory strategies of gyrase that may prove important for understanding diverse growth strategies among different bacteria.

The Microbial Toxin Microcin B17: Prospects for the Development of New Antibacterial Agents

Microcin B17 (MccB17) is an antibacterial peptide produced by strains of Escherichia coli harboring the plasmid-borne mccB17 operon. MccB17 possesses many notable features. It is able to stabilize the transient DNA gyrase-DNA cleavage complex, a very efficient mode of action shared with the highly successful fluoroquinolone drugs. MccB17 stabilizes this complex by a distinct mechanism making it potentially valuable in the fight against bacterial antibiotic resistance. MccB17 was the first compound discovered from the thiazole/oxazole-modified microcins family and the linear azole-containing peptides; these ribosomal peptides are post-translationally modified to convert serine and cysteine residues into oxazole and thiazole rings. These chemical moieties are found in many other bioactive compounds like the vitamin thiamine, the anti-cancer drug bleomycin, the antibacterial sulfathiazole and the antiviral nitazoxanide. Therefore, the biosynthetic machinery that produces these azole rings is noteworthy as a general method to create bioactive compounds. Our knowledge of MccB17 now extends to many aspects of antibacterial-bacteria interactions: production, transport, interaction with its target, and resistance mechanisms; this knowledge has wide potential applicability. After a long time with limited progress on MccB17, recent publications have addressed critical aspects of MccB17 biosynthesis as well as an explosion in the discovery of new related compounds in the thiazole/oxazole-modified microcins/linear azole-containing peptides family. It is therefore timely to summarize the evidence gathered over more than 40 years about this still enigmatic molecule and place it in the wider context of antibacterials.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Unique post-translational oxime formation in the biosynthesis of the azolemycin complex of novel ribosomal peptides from Streptomyces sp. FXJ1.264

Streptomycetes are a rich source of bioactive specialized metabolites, including several examples of the rapidly growing class of ribosomally-biosynthesized and post-translationally-modified peptide (RiPP) natural products. Here we report the discovery from Streptomyces sp. FXJ1.264 of azolemycins A-D, a complex of novel linear azole-containing peptides incorporating a unique oxime functional group. Bioinformatics analysis of the Streptomyces sp. FXJ1.264 draft genome sequence identified a cluster of genes that was hypothesized to be responsible for elaboration of the azolemycins from a ribosomally-biosynthesized precursor. Inactivation of genes within this cluster abolished azolemycin production, consistent with this hypothesis. Moreover, mutants lacking the azmE and azmF genes accumulated azolemycin derivatives lacking the O-methyl groups and an amino group in place of the N-terminal oxime (as well as proteolysed derivatives), respectively. Thus AzmE, a putative S-adenosyl methionine-dependent methyl transferase, is responsible for late-stage O-methylation reactions in azolemycin biosynthesis and AzmF, a putative flavin-dependent monooxygenase, catalyzes oxidation of the N-terminal amino group in an azolemycin precursor to the corresponding oxime. To the best of our knowledge, oxime formation is a hitherto unknown posttranslational modification in RiPP biosynthesis.

Synthesis of full length and truncated microcin B17 analogues as DNA gyrase poisons

Using a combination of solid-phase peptide synthesis and fragment assembly strategies a library of full-length and truncated analogues of the antibacterial post-translationally modified peptide microcin B17 have been synthesised. Both antibacterial and DNA gyrase poisoning activities are also described for the synthetic analogues.

Ribosomally synthesized and post-translationally modified peptide natural products: overview and recommendations for a universal nomenclature

This review presents recommended nomenclature for the biosynthesis of ribosomally synthesized and post-translationally modified peptides (RiPPs), a rapidly growing class of natural products. The current knowledge regarding the biosynthesis of the >20 distinct compound classes is also reviewed, and commonalities are discussed.

Evolution of cyclic peptide protease inhibitors

We report a bacterial system for the evolution of cyclic peptides that makes use of an expanded set of amino acid building blocks. Orthogonal aminoacyl-tRNA synthetase/tRNA(CUA) pairs, together with a split intein system were used to biosynthesize a library of ribosomal peptides containing amino acids with unique structures and reactivities. This peptide library was subsequently used to evolve an inhibitor of HIV protease using a selection based on cellular viability. Two of three cyclic peptides isolated after two rounds of selection contained the keto amino acid p-benzoylphenylalanine (pBzF). The most potent peptide (G12: GIXVSL; X=pBzF) inhibited HIV protease through the formation of a covalent Schiff base adduct of the pBzF residue with the ε-amino group of Lys 14 on the protease. This result suggests that an expanded genetic code can confer an evolutionary advantage in response to selective pressure. Moreover, the combination of natural evolutionary processes with chemically biased building blocks provides another strategy for the generation of biologically active peptides using microbial systems.

Total synthesis of microcin B17 via a fragment condensation approach

The total synthesis of the 43 amino acid antibacterial peptide Microcin B17 (MccB17) is described. The natural product was synthesized via a convergent approach from a heterocycle-derived peptide and peptide thioester fragments prepared via Fmoc-strategy solid phase peptide synthesis (SPPS). Final assembly was achieved in an efficient manner using two Ag(I)-assisted peptide ligation reactions to afford MccB17 in excellent overall yield.

Follow the leader: the use of leader peptides to guide natural product biosynthesis

The avalanche of genomic information in the past decade has revealed that natural product biosynthesis using the ribosomal machinery is much more widespread than originally anticipated. Nearly all of these compounds are crafted through post-translational modifications of a larger precursor peptide that often contains the marching orders for the biosynthetic enzymes. We review here the available information for how the peptide sequences in the precursors govern the post-translational tailoring processes for several classes of natural products. In addition, we highlight the great potential these leader peptide-directed biosynthetic systems offer for engineering conformationally restrained and pharmacophore-rich products with structural diversity that greatly expands the proteinogenic repertoire.

How nature morphs peptide scaffolds into antibiotics

The conventional notion that peptides are poor candidates for orally available drugs because of protease-sensitive peptide bonds, intrinsic hydrophilicity, and ionic charges contrasts with the diversity of antibiotic natural products with peptide-based frameworks that are synthesized and utilized by Nature. Several of these antibiotics, including penicillin and vancomycin, are employed to treat bacterial infections in humans and have been best-selling therapeutics for decades. Others might provide new platforms for the design of novel therapeutics to combat emerging antibiotic-resistant bacterial pathogens.

Gene-encoded antibiotics made in bacteria

Production of antimicrobial peptides and proteins is very common among bacteria and a variety of such substances has been described. In general Gram-negative bacteria produce protein bacteriocins (e.g. colicins) with narrow action spectra based on receptor-mediated activity. They produce comparatively few peptides, such as the post-translationally modified microcin B17. In contrast Gram-positive bacteria tend to produce peptide bacteriocins smaller than 10 kDa and of wider activity spectra. These show particular potential for application. They can be divided into unmodified peptides (e.g. lactococcins, lactacins, pediocins) and lanthionine-containing peptides (lantibiotics, e.g. nisin, epidermin, Pep5). The unmodified peptides are mostly hydrophobic or amphiphilic and act by disturbing the function of the cytoplasmic membrane. They are synthesized as prepeptides with a characteristic N-terminal leader peptide. In some cases genes for immunity peptides were found in close proximity to structural genes; furthermore, two-component response regulators seem to be involved in the regulation of their synthesis. The biosynthetic genes for lantibiotics are also organized in operons. Lantibiotic gene clusters include genes encoding the unique enzymes which dehydrate serine and threonine and form the characteristic thioether-bridged lanthionines. Three types of lantibiotics are currently distinguished on the basis of structural features and functional aspects: type A, which include elongated, amphiphilic, pore-forming peptides (e.g. nisin); type B, which are of globular shape and inhibit phospholipases (e.g. duramycins); and type C (e.g. actagardine) with intermediate features which act by inhibiting bacterial cell wall biosynthesis.

Microcins, gene-encoded antibacterial peptides from enterobacteria

Microcins are gene-encoded antibacterial peptides, with molecular masses below 10 kDa, produced by enterobacteria. They are secreted under conditions of nutrient depletion and exert potent antibacterial activity against closely related species. Typical gene clusters encoding the microcin precursor, the self-immunity factor, the secretion proteins and frequently the post-translational modification enzymes are located either on plasmids or on the chromosome. In contrast to most of the antibiotics of microbial origin, which are non-ribosomally synthesized by multimodular enzymes termed peptide synthetases, microcins are ribosomally synthesized as precursors, which are further modified enzymatically. They form a restricted class of potent antibacterial peptides. Fourteen microcins have been reported so far, among which only seven have been isolated and characterized. Despite the low number of known representatives, microcins exhibit a diversity of structures and antibacterial mechanisms. This review provides an updated overview of microcin structures, antibacterial activities, genetic systems and biosyntheses, as well as of their mechanisms of action.

Synthesis and Antibacterial Activity oftert-Butyl [1-benzyl-2[(4-aryl-2-thiazolyl)hydrazono]ethyl]carbamate Derivatives

The increasing clinical importance of drug-resistant fungal and bacterial pathogens has lent additional urgency to microbiological research and new antibacterial compound development. For this purpose, new tert-butyl[1-benzyl-2[(4-aryl-2-thiazolyl)hydrazono]ethyl]carbamate derivatives were synthesized and evaluated for antibacterial activity. The reaction of Boc-L-phenylalaninal with thiosemicarbazide gave the thiosemicarbazone which furnished the title compounds by reaction with phenacyl bromides. The newly synthesized compounds were screened for antibacterial activity and toxicity. While microdilution broth susceptibility assay was used for the antibacterial activity evaluation of the compounds against the strains E. coli (NRRL B-3704), M. luteus (NRRL B-4375), B. cereus (NRRL B-3711), P. aeruginosa (NRRL B-23), and S. fecalis (NRRL B-14617), the Artemia salina 96-well assay was used to determine cytotoxicities of the compounds. Observations obtained from the bioassays showed that some of the compounds are highly active against E. coli, M. luteus, and B. cereus when compared with the control agent and showed low toxicity.

Focus on modified microcins: structural features and mechanisms of action

Microcins are gene-encoded antimicrobial (poly)peptides secreted by Enterobacteriaceae. Produced under conditions of nutrient depletion, they are active against phylogenetically related microbial strains. Therefore, they are considered to play an important role in the microbial competitions within the intestinal flora. Among the limited sample of nine microcins hitherto described, a wide variety of structures and modes of action could be identified. The knowledge on microcins is very uneven, some being extensively studied, and others remaining uncharacterized. In this article, we have focused on a subgroup of highly modified microcins that show very original structures. We present an updated overview on the structures and mechanisms of action of microcins B17, C7 and J25, and on the associated effector proteins, also encoded by the microcin genetic system, which include specific modification enzymes, export proteins, and immunity factors.

Synthesis of thiazolines linked to a difluoromethylphosphonate diester via dithioester chemistry

[reaction: see text] A two-step, high-yielding synthesis of delta(2)-thiazolines containing a difluoromethylphosphonate diester moiety has been devised using a building block approach. Racemic or chiral beta-amino alcohols and diols were coupled with methyl difluoro(diethoxyphosphono)dithioacetate to give predominantly the corresponding beta-hydroxythioamides, which were then cyclized to provide a series of novel substituted delta(2)-thiazolines.

Titanium(IV)-mediated tandem deprotection-cyclodehydration of protected cysteine N-amides: biomimetic syntheses of thiazoline- and thiazole-containing heterocycles

The scope and limitations of TiCl(4)-mediated Delta(2)-thiazoline synthesis via tandem deprotection-dehydrocyclization of trityl-protected cysteine N-amides is presented. While chemical yields are acceptable (53-96%), the stereochemical outcomes vary on the basis of structural considerations and reaction conditions (22-99% ee). Racemization at the C(2)-exomethine position limits the utility of this method for the formation of a thiazoline within a peptide. Treatment of a tritylated Cys-Cys dipeptide with TiCl(4) afforded the corresponding thiazole-thiazoline heterocycle 12 (38% yield, 97% ee).

Lantibiotics and microcins: polypeptides with unusual chemical diversity

Bacterial-derived antimicrobial polypeptides enjoy a large degree of structural and chemical diversity. Two well-studied examples of such polypeptides are the lanthionine-containing lantibiotics produced by a variety of Gram-positive bacteria, and their Gram-negative counterparts, the microcins. Both groups are produced as gene-encoded precursor peptides and undergo post-translational modification to generate the active moieties. Structure elucidation of novel lantibiotics and microcins has recently uncovered further novel structural and chemical features and, combined with the generation of analogue peptides by genetic manipulation, new insights into structure-function relationships have been gained. Furthermore, study of the mode of action of the lantibiotics nisin and mersacidin has revealed their use of a 'docking molecule' in the target cell to facilitate their biological activities. Meanwhile, in vitro studies with microcin B17 have helped to uncover the molecular mechanisms by which post-translational modification results in the formation of heterocyclic oxazole and thiazole rings. From a practical standpoint, both groups of polypeptides represent new lead structures for future development of antimicrobial agents, whilst the identification of the 'docking molecules' represents a step forward in the search for novel targets for future antibiosis.

The structural gene for microcin H47 encodes a peptide precursor with antibiotic activity

Microcin H47 is a bactericidal antibiotic produced by a naturally occurring Escherichia coli strain isolated in Uruguay. The microcin genetic system is located in the chromosome and extends over a 10-kb DNA segment containing the genes required for microcin synthesis, secretion, and immunity. The smallest microcin synthesis gene, mchB, was sequenced and shown to encode a highly hydrophobic peptide. An mchB-phoA gene fusion, which directed the synthesis of a hybrid bifunctional protein with both PhoA and microcin H47-like activities, was isolated. The results presented herein lead us to propose that microcin H47 is indeed a ribosomally synthesized peptide antibiotic and that its peptide precursor already has antibiotic activity of the same specificity as that of mature microcin.

The cyclic structure of microcin J25, a 21-residue peptide antibiotic from Escherichia coli

Microcin J25 (MccJ25) is the single representative of the immunity group J of the microcin group of peptide antibiotics produced by Enterobacteriaceae. It induces bacterial filamentation in susceptible cells in a non-SOS-dependent pathway [R. A. Salomon and R. Farias (1992) J. Bacteriol. 174, 7428-7435]. MccJ25 was purified to homogeneity from the growth medium of a microcin-overproducing Escherichia coli strain by reverse-phase HPLC. Based on amino acid composition and absolute configuration determination, liquid secondary ion and electrospray mass spectrometry, extensive two-dimensional NMR, enzymatic and chemical degradations studies, the structure of MccJ25 was elucidated as a 21-residue peptide, cyclo(-Val1-Gly-Ile-Gly-Thr- Pro-Ile-Ser-Phe-Tyr-Gly-Gly-Gly-Ala-Gly-His-Val-Pro-Glu-Tyr-Phe21- ). Although MccJ25 showed high resistance to most of endoproteases, linearization by thermolysin occurred from cleavage at the Phe21-Val1 bond and led to a single peptide, MccJ25-L. While MccJ25 exhibited remarkable antibiotic activity towards Salmonella newport and several E. coli strains (minimal inhibitory concentrations ranging between 0.01 and 0.2 microgram.mL-1), the thermolysin-linearized microcin showed a dramatic decrease of the activity, indicating that the cyclic structure is essential for the MccJ25 biological properties. As MccJ25 is ribosomally synthesized as a larger peptide precursor endowed with an N-terminal extremity, the present study shows that removal of this extension and head-tail cyclization of the resulting propeptide are the only post-translational modifications involved in the maturation of MccJ25, that appears as the first cyclic microcin.

Role of the microcin B17 propeptide in substrate recognition: solution structure and mutational analysis of McbA1-26

BACKGROUND

The peptide antibiotic microcin B17 (MccB17) contains oxazole and thiazole heterocycles formed by the post-translational modification of four cysteine and four serine residues. An amino-terminal propeptide targets the 69 amino acid precursor of MccB17 (preproMccB17) to the heterocyclization enzyme MccB17 synthetase. The mode of synthetase recognition has been unclear, because there has been limited structural information available on the MccB17 propeptide to date.

RESULTS

The solution structure of the MccB17 propeptide (McbA1-26), determined using nuclear magnetic resonance, reveals that McbA1-26 is an amphipathic alpha helix. Mutational analysis of 13 propeptide residues showed that Phe8 and Leu12 are essential residues for MccB17 synthetase recognition. A domain of the propeptide was putatively identified as the region that interacts with the synthetase.

CONCLUSIONS

MccB17 synthetase recognizes key hydrophobic residues within a helical propeptide, allowing the selective heterocyclization of downstream cysteine and serine residues in preproMccB17. The determination of the solution structure of the propeptide should facilitate the investigation of other functions of the propeptide, including a potential role in antibiotic secretion.

The enethiolate anion reaction products of EpiD. Pka value of the enethiol side chain is lower than that of the thiol side chain of peptides

One of the steps involved in the biosynthesis of the lantibiotic epidermin is the oxidative decarboxylation reaction of peptides catalyzed by the flavoenzyme EpiD. EpiD catalyzes the formation of a (Z)-enethiol derivative from the C-terminal cysteine residue of the precursor peptide of epidermin and related peptides. The UV-visible spectra of the reaction products of EpiD are pH-dependent, indicating that the enethiol side chain is converted to an enethiolate anion. The pKa value of the enethiol group was determined to be 6.0 and is substantially lower than the pKa value of the thiol side chain of cysteine residues. The increased acid strength of the enethiol side chain compared with that of the thiol group is attributed to the resonance stabilization of the negative charge of the anion.

Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants

The Rhizobium meliloti bacA gene encodes a function that is essential for bacterial differentiation into bacteroids within plant cells in the symbiosis between R. meliloti and alfalfa. An Escherichia coli homolog of BacA, SbmA, is implicated in the uptake of microcin B17, microcin J25 (formerly microcin 25), and bleomycin. When expressed in E. coli with the lacZ promoter, the R. meliloti bacA gene was found to suppress all the known defects of E. coli sbmA mutants, namely, increased resistance to microcin B17, microcin J25, and bleomycin, demonstrating the functional similarity between the two proteins. The R. meliloti bacA386::Tn(pho)A mutant, as well as a newly constructed bacA deletion mutant, was found to show increased resistance to bleomycin. However, it also showed increased resistance to certain aminoglycosides and increased sensitivity to ethanol and detergents, suggesting that the loss of bacA function causes some defect in membrane integrity. The E. coli sbmA gene suppressed all these bacA mutant phenotypes as well as the Fix- phenotype when placed under control of the bacA promoter. Taken together, these results strongly suggest that the BacA and SbmA proteins are functionally similar and thus provide support for our previous hypothesis that BacA may be required for uptake of some compound that plays an important role in bacteroid development. However, the additional phenotypes of bacA mutants identified in this study suggest the alternative possibility that BacA may be needed for membrane integrity, which is likely to be critically important during the early stages of bacterial differentiation within plant cells.

From peptide precursors to oxazole and thiazole-containing peptide antibiotics: microcin B17 synthase

Esherichia coli microcin B17 is a posttranslationally modified peptide that inhibits bacterial DNA gyrase. It contains four oxazole and four thiazole rings and is representative of a broad class of pharmaceutically important natural products with five-membered heterocycles derived from peptide precursors. An in vitro assay was developed to detect heterocycle formation, and an enzyme complex, microcin B17 synthase, was purified and found to contain three proteins, McbB, McbC, and McbD, that convert 14 residues into the eight mono- and bisheterocyclic moieties in vitro that confer antibiotic activity on mature microcin B17. These enzymatic reactions alter the peptide backbone connectivity. The propeptide region of premicrocin is the major recognition determinant for binding and downstream heterocycle formation by microcin B17 synthase. A general pathway for the enzymatic biosynthesis of these heterocycles is formulated.

Total Synthesis of (-)-Muscoride A

The recently isolated cyanobacterium metabolite muscoride A was synthesized in 15 steps and in 4.3% overall yield. Novel structural features of this peptide antibiotic include the presence of a threonine-derived bioxazole core and an N-(1,1-dimethyl)allyl ("reverse prenyl") valine residue. In the context of our synthesis, efficient new strategies for the preparation of these segments were developed. The synthesis of two epimers of muscoride A allowed the unambiguous assignment of the relative and absolute configuration of the natural product by NMR and optical rotation analyses.

Post-translational modifications of lantibiotics

Several newly reported post-translational modification reactions are involved in lantibiotic biosynthesis. A short overview of the present knowledge on the post-translational modifications and on the enzymes involved in lantibiotic biosynthesis is given. The oxidative decarboxylation of the epidermin precursor peptide EpiA is described in detail. The FMN-containing oxidoreductase EpiD is involved in the formation of the C-terminal S-[(Z)-2-aminovinyl]-D-cysteine residue of epidermin: under reducing conditions the side chain of the C-terminal cysteine residue of EpiA is converted to an enethiol. EpiD has no absolute substrate specificity and can be used for modification of peptides having the C-terminal consensus motif [V/I/L/(M)/F/Y/W]-[A/S/V/T/C/(I/L)]-C.

Genetic analysis of Rhizobium meliloti bacA-phoA fusion results in identification of degP: two loci required for symbiosis are closely linked to degP

The function of the Rhizobium meliloti bacA gene, which is a homolog of the Escherichia coli sbmA gene, is required for an intermediate step in nodule development. A strain carrying the bacA386::TnphoA fusion was mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine, and three mutants that had higher levels of alkaline phosphatase activity were identified. The mutations in these strains were recessive and mapped to the same genetic locus. The gene affected by these mutations was identified and sequenced and was found to be a homolog of the E. coli degP gene, which encodes a periplasmic endopeptidase. Although degP function is important for the virulence of certain intracellular pathogens of mammals, it is not required for the R. meliloti-alfalfa symbiosis. The genetic analyses involving degP were complicated by the presence of a locus immediately upstream of depP that was lethal when present in multiple copies in a DegP- background. R. meliloti derivatives carrying insertion mutations in this locus displayed an N,N,N',N'-tetramethyl-p-phenylenediamine oxidase-negative phenotype, elicited the formation of white cylindrical nodules that did not fix nitrogen, and grew slowly in rich medium, suggesting that the locus was a cyc gene encoding a protein involved in the biosynthesis of a component or components of a respiratory chain. The previously identified fix-382::TnphoA, which similarly causes the formation of white cylindrical nodules that do not fix nitrogen, was shown to affect a gene that is separate from this cyc gene but extremely closely linked to it.

Post-translational heterocyclic backbone modifications in the 43-peptide antibiotic microcin B17. Structure elucidation and NMR study of a 13C,15N-labelled gyrase inhibitor

Microcin B17 (McB17), the first known gyrase inhibitor of peptidic nature, is produced by ribosomal synthesis and post-translational modification of the 69-residue precursor protein by an Escherichia coli strain. To elucidate the chemical structure of the mature 43-residue peptide antibiotic, fermentation and purification protocols were established and optimized which allowed the isolation and purification of substantial amounts of highly pure McB17 (non-labelled, 15N-labelled and 13C/15N-labelled peptide. By ultraviolet-absorption spectroscopy. HPLC-electrospray mass spectrometry and GC-mass spectrometry, amino acid analysis, protein sequencing, and, in particular, multidimensional NMR, we could demonstrate and unequivocally prove that the enzymic modification of the precursor backbone at Gly-Cys and Gly-Ser segments leads to the formation of 2-aminomethylthiazole-4-carboxylic acid and 2-aminomethyloxazole-4-carboxylic acid, respectively. In addition, two bicyclic modifications 2-(2-aminomethyloxazolyl)thiazole-4-carboxylic acid and 2-(2-aminomethylthiazolyl)oxazole-4-carboxylic acid were found that consist of directly linked thiazole and oxazole rings derived from one Gly-Ser-Cys and one Gly-Cys-Ser segment. Analogous to the thiazole and oxazole rings found in antitumor peptides of microbial and marine origin, these heteroaromatic ring systems of McB17 presumably play an important role in its gyrase-inhibiting activity, e.g. interacting with the DNA to trap the covalent protein-DNA intermediate of the breakage-reunion reaction of the gyrase.

Biosynthesis and biological activities of lantibiotics with unique post-translational modifications

Lantibiotics are biologically active peptides which contain the thioether amino acid lanthionine as well as several other modified amino acids. They can be broadly divided into two groups on the basis of their structures: type-A lantibiotics are elongated, amphiphilic peptides, while type-B lantibiotics are compact and globular. In the last decade there has been a marked increase in research interest in these peptides due both to the novel biosynthetic mechanisms by which they are produced, as well as to their potential applications. Lantibiotics are synthesised on the ribosome as a prepeptide which undergoes several post-translational modification events, including dehydration of specific hydroxyl amino acids to form dehydroamino acids, addition of neighbouring sulfhydryl groups to form thioethers and, in specific cases, other modifications such as introduction of D-alanine residues from L-serine, formation of lysinoalanine bridges, formation of novel N-terminal blocking groups and oxidative decarboxylation of a C-terminal cysteine. The genetic elements responsible for these specific modification reactions encode unique enzymes with hitherto unknown reaction mechanisms. Production of these peptides also requires accessory proteins including processing proteases, translocators of the ATP-binding cassette transporter family, regulatory proteins and dedicated producer self-protection mechanisms. While the principle biological activity of most type-B lantibiotics appears to be directed at the inhibition of enzyme functions, the type-A lantibiotics kill bacterial cells by forming pores in the cytoplasmic membrane.

Structure of microcin C51, a new antibiotic with a broad spectrum of activity

The structure of microcin C51, a new antibiotic produced by E. coli, has been determined. This antibiotic was shown to be a 1.18 kDa nucleotide peptide. It consists of a heptapeptide with formylmethionine as the N-terminus and a C-terminal asparagine linked with nebularin-5'-monophosphate through the three-methylene bridge. The OH-group of threonine is substituted. The peptide chain of microcin C51 synthesized on ribosomes is the longest among the known biologically active nucleotide peptides.