We identified a triazole–quinoline derivative (compound 7) of a 16-membered macrolide that exhibits enhanced antibacterial activity against Gram-negative pathogens. While the parent compound OMT showed limited activity, compound 7 exhibited enhanced potency, which was further markedly potentiated by the clinically evaluated outer membrane permeabilizer SPR741. In the presence of SPR741, MIC values of compound 7 were reduced to ≤1 μg/mL against multiple Gram-negative species, including multidrug-resistant Escherichia coli and Klebsiella pneumoniae carrying macrolide resistance determinants. Synergistic interactions were confirmed (FIC ≤ 0.5). Importantly, the combination demonstrated therapeutic efficacy in a silkworm infection model (ED50 = 42 mg/kg). These findings support a drug repurposing strategy in which structurally optimized macrolides, combined with clinically relevant adjuvants, can overcome intrinsic and acquired resistance in Gram-negative pathogens, highlighting their translational potential.
Post-translational modifications (PTMs) of peptides are promising approaches for introducing desired chemical and biological properties. Nature often employs this strategy in the biosynthesis of bioactive peptidic natural products, which acquire their unique structures and functions through diverse enzymatic conversions. Ribosomally synthesized and post-translationally modified peptides (RiPPs) represent a prominent example and constitute a major group of peptidic natural products generated through various PTMs of ribosomally translated precursor peptides. Owing to their biosynthetic features, RiPP PTM enzymes provide a valuable foundation for peptide bioengineering. Numerous engineering approaches have been developed and have increasingly expanded beyond native RiPP substrates to artificial peptides that substantially diverge from them. Here, we review recent bioengineering approaches that exploit RiPP PTM enzymes for the modification of artificial peptides, including large combinatorial peptide libraries, de novo designed peptides, and chemically synthesized peptides. These advances demonstrate the potential of RiPP biosynthetic machinery as a versatile toolkit for expanding peptide chemical space and generating bioactive artificial peptides.
5-O-desmethyl kinanthraquinone B, a new kinanthraquinone analogue, was isolated from a kiqT1 disruptant. Its structure was elucidated by high-resolution electrospray ionisation mass spectrometry and nuclear magnetic resonance spectrometry and subsequently compared with the spectroscopic data of kinanthraquinone B. The newly isolated analogue exhibited antibacterial activity against Staphylococcus aureus, antimalarial activity against Plasmodium falciparum, and cytotoxicity toward HL-60 cells, with IC₅₀ values of 3.7, 5.5 and 11 μg/mL, respectively. Comparison of the biological activities of 5-O-desmethyl kinanthraquinone B and kinanthraquinone B indicated that methylation of the C-5 hydroxyl group reduced antibacterial activity and cytotoxicity with little effect on antimalarial activity.
Triculamin is a potent antibiotic lasso peptide first isolated in 1967. Previous studies have demonstrated that its biosynthesis follows a non-canonical logic unlike any other lasso peptide. In this study, we investigate the role of the unusual follower peptide and demonstrate that it is essential for biosynthesis. Using structural prediction and targeted mutations of key conserved residues, we hypothesize that the interactions between the follower peptide and the macrocyclase create an enzyme-substrate complex that ensures delivery of the core peptide to the enzyme active site. Moreover, we demonstrate that analogs of the lasso peptide can be produced by modifying the core peptide, highlighting the substrate promiscuity of the lasso macrocyclase and identifying lysine-3 in the lasso peptide ring as the site of acetylation. Lastly, we achieved successful heterologous expression in Burkholderia sp. FERM BP-3421, which proves to be a superior heterologous host.
Through the intensive investigation of the chemical components for the culture broth of marine-derived Bacillus sp. ROA-056, two new linear peptides, namely, pobacilamides A (1) and B (2), have been discovered. Their planar structures were elucidated through mass spectrometry, and 1D and 2D NMR analyses. Both peptides were composed of valine (Val), 2-hydroxyisovaleric acid (HIVA), and 2-hydroxyisocaproic acid (HICA), while pobacilamide A (1) contained leucine (Leu). The absolute configurations of Val and Leu were defined by acid hydrolysis and advanced Marfey's method, while those of HIVA and HICA were established by GC-MS analysis of O-trifluoroacetylated (S)-(+)-3-methyl-2-butyl ester. Compounds 1 and 2 displayed selective antibacterial activity against Gram-positive bacteria, including Bacillus subtilis and Kocuria rhizophila, with minimum inhibitory concentrations (MICs) values from 0.25 mu g/mL to 4 mu g/mL. Pobacilamide B (2) showed the strongest activity against K. rhizophila with an MIC of 0.25 mu g/mL. No antibacterial activity was observed against Gram-negative strains. These results suggested that compounds 1 and 2 have considerable potential as targeted antibacterial agents for Gram-positive pathogens.
