Microcin B17 blocks DNA replication and induces the SOS system in Escherichia coli

Microcin B17 is a novel peptide antibiotic of low Mr (about 4000) produced by Escherichia coli strains carrying plasmid pMccB17. The action of this microcin in sensitive cells is essentially irreversible, follows single-hit kinetics, and leads to an abrupt arrest of DNA replication and, consequently, to the induction of the SOS response. RecA- and RecBC- strains are hypersensitive to microcin B17. Strains producing a non-cleavable SOS repressor (lexAl mutant) are also more sensitive than wild-type, whereas strains carrying a mutation which causes constitutive expression of the SOS response (spr-55) are less sensitive to microcin. Microcin B17 does not induce the SOS response in cells which do not have an active replication fork. The results suggest that the mode of action of this microcin is different from all other well-characterized microcins and colicins, and from other antibiotics which inhibit DNA replication.     

Evidence that colicin X is microcin B17.

The DNA replication inhibitor microcin B17 is a peptide antibiotic produced by Escherichia coli strains carrying plasmid pMccB17. Here we present evidence that antibiotic activities previously named colicin X are probably identical to microcin B17. Our results include comparison of the conditions of production of the antibiotics, their mode of action, cross-immunity of producer strains, and cross-resistance of resistant mutants. Plasmids encoding colicin X have been identified and shown to have a region of significant homology with the microcin B17-producing region of pMccB17 DNA.     

Plasmid genes required for microcin B17 production.

The production of the antibiotic substance microcin B17 (Mcc) is determined by a 3.5-kilobase DNA fragment from plasmid pMccB17. Several Mcc- mutations on plasmid pMccB17 were obtained by both transposon insertion and nitrosoguanidine mutagenesis. Plasmids carrying these mutations were tested for their ability to complement Mcc- insertion or deletion mutations on pMM102 (pMM102 is a pBR322 derivative carrying the region encoding microcin B17). Results from these experiments indicate that at least four plasmid genes are required for microcin production.     

The antibiotic microcin B17 is a DNA gyrase poison: characterisation of the mode of inhibition

Microcin B17 is a 3.1-kDa bactericidal peptide; the putative target of this antibiotic is DNA gyrase. Microcin B17 has no detectable effect on gyrase-catalysed DNA supercoiling or relaxation activities in vitro and is unable to stabilise DNA cleavage in the absence of nucleotides. However, in the presence of ATP, or the non-hydrolysable analogue 5'-adenylyl beta,gamma-imidodiphosphate, microcin B17 stabilises a gyrase-dependent DNA cleavage complex in a manner reminiscent of quinolones, Ca(2+), or the bacterial toxin CcdB. The pattern of DNA cleavage produced by gyrase in the presence of microcin B17 is different from that produced by quinolones and more closely resembles Ca(2+)-mediated cleavage. Several gyrase mutants, including well-known quinolone-resistant mutants, are cross resistant to microcin-induced DNA cleavage. We suggest that microcin exerts its effects through a mechanism that has similarities to those of both the bacterial toxin CcdB and the quinolone antibacterial agents.     

Cloning and mapping of the genetic determinants for microcin B17 production and immunity.

Plasmid pMccB17 (70 kilobases [kb]) codes for the production of microcin B17, a peptide that inhibits DNA synthesis, and for microcin B17 immunity. A BamHI-EcoRI fragment of 5.1 kb from pMccB17 was cloned into pBR322 in two steps. The resulting plasmid (pMM102) overproduced microcin B17 and expressed immunity against microcin. Mcc- and Mcc- Imm- mutants were isolated on plasmids pMccB17 and pMM102 by deleting various DNA fragments and by inserting different translocatable elements. Physical and phenotypic characterization of these mutants showed that a DNA region of 3.0 to 3.5 kb is required to produce microcin B17, whereas an adjacent region of about 1.0 kb is required to express microcin B17 immunity.     

Shear stress enhances microcin B17 production in a rotating wall bioreactor, but ethanol stress does not

Stress, including that caused by ethanol, has been shown to induce or promote secondary metabolism in a number of microbial systems. Rotating-wall bioreactors provide a low stress and simulated microgravity environment which, however, supports only poor production of microcin B17 by Escherichia coli ZK650, as compared to production in agitated flasks. We wondered whether the poor production is due to the low level of stress and whether increasing stress in the bioreactors would raise the amount of microcin B17 formed. We found that applying shear stress by addition of a single Teflon bead to a rotating wall bioreactor improved microcin B17 production. By contrast, addition of various concentrations of ethanol to such bioreactors (or to shaken flasks) failed to increase microcin B17 production. Ethanol stress merely decreased production and, at higher concentrations, inhibited growth. Interestingly, cells growing in the bioreactor were much more resistant to the growth-inhibitory and production-inhibitory effects of ethanol than cells growing in shaken flasks.     

The action of the bacterial toxin, microcin B17, on DNA gyrase

Microcin B17 (MccB17) is a peptide-based bacterial toxin that targets DNA gyrase, the bacterial enzyme that introduces supercoils into DNA. The site and mode of action of MccB17 on gyrase are unclear. We review what is currently known about MccB17-gyrase interactions and summarise approaches to understanding its mode of action that involve modification of the toxin. We describe experiments in which treatment of the toxin at high pH leads to the deamidation of two asparagine residues to aspartates. The modified toxin was found to be inactive in vivo and in vitro, suggesting that the Asn residues are essential for activity. Following on from these studies we have used molecular modelling to suggest a 3D structure for microcin B17. We discuss the implications of this model for MccB17 action and investigate the possibility that it binds metal ions.     

The DNA replication inhibitor microcin B17 is a forty-three-amino-acid protein containing sixty percent glycine

Microcin B17 is a low-molecular-weight protein that inhibits DNA replication in a number of enteric bacteria. It is produced by bacterial strains which harbor a 70-kilobase plasmid called pMccB17. Four plasmid genes (named mcbABCD) are required for its production. The product of the mcbA gene was identified by labelling minicells. The mcbA gene product was slightly larger when a mutation in any of the other three production genes was present. This indicates that these genes are involved in processing the primary mcbA product to yield the active molecule. The mcbA gene product predicted from the nucleotide sequence has 69 amino acids including 28 glycine residues. Microcin B17 was extracted from the cells by boiling in 100 mM acetic acid, 1 mM EDTA, and purified to homogeneity in a single step by high-performance liquid chromatography through a C18 column. The N-terminal amino acid sequence and amino acid composition demonstrated that mcbA is the structural gene for microcin B17. The active molecule is a processed product lacking the first 26 N-terminal residues. The 43 remaining residues include 26 glycines. While microcin B17 is an exported protein, the cleaved N-terminal peptide does not have the characteristic properties of a "signal sequence", which suggests that it is secreted by a mechanism different from that used by most secreted proteins of E. coli.     

Computational analysis of the first biheterocyclization site of the antibiotic microcin B17

Microcin B 17 (MccB17) undergoes an enzyme catalyzed posttranslational modification to form four oxazole and four thiazole rings. Four of these rings form 4,2 - connected biheterocyclic functionalities. In this study, the hexapeptide sequence surrounding the first biheterocyclization site of microcin B17 was examined using computational calculations and database analysis to see if it was preorganized for cyclization in a manner similar to that found in the autocatalytic posttranslational cyclization of Green Fluorescent Protein (GFP). Attention was focused on the intermolecular distances between the sulfur and oxygen atoms of the cysteine and serine residues and the carbonyl carbons which they attack in the ring formation. Conformational searches located some low energy conformations that contained relatively short oxygen to carbonyl carbon distances, which indicated that the oxazole forming fragment in microcin B17 is preorganized for cyclization. However, the lack of any clear patterns for the sulfur to carbon distances show that the side-chain of cysteine does not adopt any low energy conformations that are geometrically preorganized for cyclization. The MccB17 synthetase enzyme complex which catalyzes the cyclization process therefore has both steric and electronic functions. The data obtained in this investigation is in agreement with empirical data which shows that biheterocyclization will only occur if the thiazole forms before the oxazole.     

Relief from glucose interference in microcin B17 biosynthesis by growth in a rotating-wall bioreactor

Glucose interference in production of microcin B17 by Escherichia coli ZK650 was decreased sevenfold by growth in a ground-based rotating-wall bioreactor operated in the simulated microgravity mode as compared with growth in flasks. When cells were grown in the bioreactor in the normal gravity mode, relief from glucose interference was even more dramatic, amounting to a decrease in glucose interference of over 100-fold.     

The maturation pathway of microcin B17, a peptide inhibitor of DNA gyrase

The maturation pathway of microcin B17 (MccB17), a ribosomally synthesized peptide antibiotic which inhibits DNA gyrase, has been characterized. Synthesis of MccB17 involves several steps beginning with the translation of the MccB17 structural gene, mcbA, to yield a 69 amino acid precursor, preMccB17. PreMccB17 is then modified and folded by the action of three gene products, McbBCD, to yield proMccB17. Mutations in mcbA were isolated that permit modifications of the resulting mutant peptides, but prevent folding, suggesting that modification and folding are sequential steps. ProMccB17 is subsequently converted to MccB17 by removal of the W-terminal 26-amino-acid leader by a chromosomally encoded protease. Removal of the leader resulted in aggregation of the peptide, suggesting that the leader may function to maintain peptide solubility during synthesis in the cell. Finally, polyclonal antibodies raised against MccB17 recognize both MccB17 and proMccB17, but do not recognize preMccB17. This demonstrates the dramatic structural changes that result from the modifications and has been used to distinguish intermediates in the steps of maturation.     

Effect of simulated microgravity and shear stress on microcin B17 production by Escherichia coli and on its excretion into the medium.

Production of the antibacterial polypeptide microcin B17 (MccB17) by Escherichia coli ZK650 was inhibited by simulated microgravity. The site of MccB17 accumulation was found to be different, depending on whether the organism was grown in shaking flasks or in rotating bioreactors designed to establish a simulated microgravity environment. In flasks, the accumulation was cellular, but in the reactors, virtually all the microcin was found in the medium. The change from a cellular site to an extracellular one was apparently not a function of gravity, since extracellular production occurred in these bioreactors, irrespective of whether they were operated in the simulated microgravity or normal gravity mode. More probably, excretion is due to the much lower degree of shear stress in the bioreactors. Addition of even a single glass bead to the 50-ml medium volume in the bioreactor created enough shear to change the site of MccB17 accumulation from the medium to the cells.     

The action of the bacterial toxin microcin B17. Insight into the cleavage-religation reaction of DNA gyrase

We have examined the effects of the bacterial toxin microcin B17 (MccB17) on the reactions of Escherichia coli DNA gyrase. MccB17 slows down but does not completely inhibit the DNA supercoiling and relaxation reactions of gyrase. A kinetic analysis of the cleavage-religation equilibrium of gyrase was performed to determine the effect of the toxin on the forward (cleavage) and reverse (religation) reactions. A simple mechanism of two consecutive reversible reactions with a nicked DNA intermediate was used to simulate the kinetics of cleavage and religation. The action of MccB17 on the kinetics of cleavage and religation was compared with that of the quinolones ciprofloxacin and oxolinic acid. With relaxed DNA as substrate, only a small amount of gyrase cleavage complex is observed with MccB17 in the absence of ATP, whereas the presence of the nucleotide significantly enhances the effect of the toxin on both the cleavage and religation reactions. In contrast, ciprofloxacin, oxolinic acid, and Ca2+ show lesser dependence on ATP to stabilize the cleavage complex. MccB17 enhances the overall rate of DNA cleavage by increasing the forward rate constant (k2) of the second equilibrium. In contrast, ciprofloxacin increases the amount of cleaved DNA by a combined effect on the forward and reverse rate constants of both equilibria. Based on these results and on the observations that MccB17 only slowly inhibits the supercoiling and relaxation reactions, we suggest a model of the interaction of MccB17 with gyrase.     

Gene ompR and regulation of microcin 17 and colicin e2 syntheses.

The production of microcin 17 is controlled by plasmid pRYC17. Chromosomal mutants unable to produce a normal amount of microcin were isolated in Escherichia coli. One of the mutations maps in the ompR locus, indicating that an active OmpR product is required for the synthesis of microcin 17. The same conclusion was obtained for the synthesis of colicin E2. Therefore, two new functions of the regulatory gene ompR have been revealed.     

The highly conserved TldD and TldE proteins of Escherichia coli are involved in microcin B17 processing and in CcdA degradation

Microcin B17 (MccB17) is a peptide antibiotic produced by Escherichia coli strains carrying the pMccB17 plasmid. MccB17 is synthesized as a precursor containing an amino-terminal leader peptide that is cleaved during maturation. Maturation requires the product of the chromosomal tldE (pmbA) gene. Mature microcin is exported across the cytoplasmic membrane by a dedicated ABC transporter. In sensitive cells, MccB17 targets the essential topoisomerase II DNA gyrase. Independently, tldE as well as tldD mutants were isolated as being resistant to CcdB, another natural poison of gyrase encoded by the ccd poison-antidote system of plasmid F. This led to the idea that TldD and TldE could regulate gyrase function. We present in vivo evidence supporting the hypothesis that TldD and TldE have proteolytic activity. We show that in bacterial mutants devoid of either TldD or TldE activity, the MccB17 precursor accumulates and is not exported. Similarly, in the ccd system, we found that TldD and TldE are involved in CcdA and CcdA41 antidote degradation rather than being involved in the CcdB resistance mechanism. Interestingly, sequence database comparisons revealed that these two proteins have homologues in eubacteria and archaebacteria, suggesting a broader physiological role.     

Construction and characterization of mutations at codon 751 of the Escherichia coli gyrB gene that confer resistance to the antimicrobial peptide microcin B17 and alter the activity of DNA gyrase

Microcin B17 is a peptide antibiotic that inhibits DNA replication in Escherichia coli by targeting DNA gyrase. Previously, two independently isolated microcin B17-resistant mutants were shown to harbor the same gyrB point mutation that results in the replacement of tryptophan 751 by arginine in the GyrB polypeptide. We used site-directed mutagenesis to construct mutants in which tryptophan 751 was deleted or replaced by other amino acids. These mutants exhibit altered DNA gyrase activity and different levels of resistance to microcin B17.     

Plantazolicin, a novel microcin B17/streptolysin S-like natural product from Bacillus amyloliquefaciens FZB42

Here we report on a novel thiazole/oxazole-modified microcin (TOMM) from Bacillus amyloliquefaciens FZB42, a Gram-positive soil bacterium. This organism is well known for stimulating plant growth and biosynthesizing complex small molecules that suppress the growth of bacterial and fungal plant pathogens. Like microcin B17 and streptolysin S, the TOMM from B. amyloliquefaciens FZB42 undergoes extensive posttranslational modification to become a bioactive natural product. Our data show that the modified peptide bears a molecular mass of 1,335 Da and displays antibacterial activity toward closely related Gram-positive bacteria. A cluster of 12 genes that covers ～10 kb is essential for the production, modification, export, and self-immunity of this natural product. We have named this compound plantazolicin (PZN), based on the association of several producing organisms with plants and the incorporation of azole heterocycles, which derive from Cys, Ser, and Thr residues of the precursor peptide.