Plantazolicin is an ultra-narrow spectrum antibiotic that targets the Bacillus anthracis membrane.

Plantazolicin (PZN) is a ribosomally synthesized and post-translationally modified natural product from Bacillus methylotrophicus FZB42 and Bacillus pumilus. Extensive tailoring to twelve of the fourteen amino acid residues in the mature natural product endows PZN with not only a rigid, polyheterocyclic structure, but also antibacterial activity. Here we report a remarkably discriminatory activity of PZN toward Bacillus anthracis, which rivals a previously-described gamma (γ) phage lysis assay in distinguishing B. anthracis from other members of the Bacillus cereus group. We evaluate the underlying cause of this selective activity by measuring the RNA expression profile of PZN-treated B. anthracis, which revealed significant upregulation of genes within the cell envelope stress response. PZN depolarizes the B. anthracis membrane like other cell envelope-acting compounds but uniquely localizes to distinct foci within the envelope. Selection and whole-genome sequencing of PZN-resistant mutants of B. anthracis implicate a relationship between the action of PZN and cardiolipin (CL) within the membrane. Exogenous CL increases the potency of PZN in wild type B. anthracis and promotes the incorporation of fluorescently tagged PZN in the cell envelope. We propose that PZN localizes to and exacerbates structurally compromised regions of the bacterial membrane, which ultimately results in cell lysis.

Engineering unnatural variants of plantazolicin through codon reprogramming.

Plantazolicin (PZN) is a polyheterocyclic natural product derived from a ribosomal peptide that harbors remarkable antibiotic selectivity for the causative agent of anthrax, Bacillus anthracis. To simultaneously establish the structure-activity relationship of PZN and the substrate tolerance of the biosynthetic pathway, an Escherichia coli expression strain was engineered to heterologously produce PZN analogues. Variant PZN precursor genes were produced by site-directed mutagenesis and later screened by mass spectrometry to assess post-translational modification and export by E. coli. From a screen of 72 precursor peptides, 29 PZN variants were detected. This analogue collection provided insight into the selectivity of the post-translational modifying enzymes and established the boundaries of the natural biosynthetic pathway. Unlike other studied thiazole/oxazole-modified microcins, the biosynthetic machinery appeared to be finely tuned toward the production of PZN, such that the cognate enzymes did not process even other naturally occurring sequences from similar biosynthetic clusters. The modifying enzymes were exquisitely selective, installing heterocycles only at predefined positions within the precursor peptides while leaving neighboring residues unmodified. Nearly all substitutions at positions normally harboring heterocycles prevented maturation of a PZN variant, though some exceptions were successfully produced lacking a heterocycle at the penultimate residue. No variants containing additional heterocycles were detected, although several peptide sequences yielded multiple PZN variants as a result of varying oxidation states of select residues. Eleven PZN variants were produced in sufficient quantity to facilitate purification and assessment of their antibacterial activity, providing insight into the structure-activity relationship of PZN.

Antibacterial drug leads targeting isoprenoid biosynthesis.

With the rise in resistance to antibiotics such as methicillin, there is a need for new drugs. We report here the discovery and X-ray crystallographic structures of 10 chemically diverse compounds (benzoic, diketo, and phosphonic acids, as well as a bisamidine and a bisamine) that inhibit bacterial undecaprenyl diphosphate synthase, an essential enzyme involved in cell wall biosynthesis. The inhibitors bind to one or more of the four undecaprenyl diphosphate synthase inhibitor binding sites identified previously, with the most active leads binding to site 4, outside the catalytic center. The most potent leads are active against Staphylococcus aureus [minimal inhibitory concentration (MIC)(90) ∼0.25 µg/mL], and one potently synergizes with methicillin (fractional inhibitory concentration index = 0.25) and is protective in a mouse infection model. These results provide numerous leads for antibacterial development and open up the possibility of restoring sensitivity to drugs such as methicillin, using combination therapies.

HIV-1 Integrase Inhibitor-Inspired Antibacterials Targeting Isoprenoid Biosynthesis.

We report the discovery of antibacterial leads, keto- and diketo-acids, targeting two prenyl transferases: undecaprenyl diphosphate synthase (UPPS) and dehydrosqualene synthase (CrtM). The leads were suggested by the observation that keto- and diketo-acids bind to the active site Mg(2+)/Asp domain in HIV-1 integrase, and similar domains are present in prenyl transferases. We report the x-ray crystallographic structures of one diketo-acid and one keto-acid bound to CrtM, which supports the Mg(2+) binding hypothesis, together with the x-ray structure of one diketo-acid bound to UPPS. In all cases, the inhibitors bind to a farnesyl diphosphate substrate-binding site. Compound 45 had cell growth inhibition MIC(90) values of ~250-500 ng/mL against S. aureus, 500 ng/mL against Bacillus anthracis, 4 µg/mL against Listeria monocytogenes and Enterococcus faecium, and 1 µg/mL against Streptococcus pyogenes M1, but very little activity against E. coli (DH5α, K12) or human cell lines.

Structure determination and interception of biosynthetic intermediates for the plantazolicin class of highly discriminating antibiotics.

The soil-dwelling, plant growth-promoting bacterium Bacillus amyloliquefaciens FZB42 is a prolific producer of complex natural products. Recently, a new FZB42 metabolite, plantazolicin (PZN), has been described as a member of the growing thiazole/oxazole-modified microcin (TOMM) family. TOMMs are biosynthesized from inactive, ribosomal peptides and undergo a series of cyclodehydrations, dehydrogenations, and other modifications to become bioactive natural products. Using high-resolution mass spectrometry, chemoselective modification, genetic interruptions, and other spectroscopic tools, we have determined the molecular structure of PZN. In addition to two conjugated polyazole moieties, the amino-terminus of PZN has been modified to N(α),N(α)-dimethylarginine. PZN exhibited a highly selective antibiotic activity toward Bacillus anthracis, but no other tested human pathogen. By altering oxygenation levels during fermentation, PZN analogues were produced that bear variability in their heterocycle content, which yielded insight into the order of biosynthetic events. Lastly, genome-mining has revealed the existence of four additional PZN-like biosynthetic gene clusters. Given their structural uniqueness and intriguing antimicrobial specificity, the PZN class of antibiotics may hold pharmacological value.

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.