Unraveling key steps in the biosynthesis of antimicrobial methylated unsaturated 2-alkyl-4-quinolones of Burkholderia thailandensis

The 2-alkyl-4(1H)-quinolone family of natural products comprises a diverse set of compounds acting as signals and antibiotics. The 2-alkyl-4(1H)-quinolone biosynthetic pathway of Burkholderia thailandensis exhibits a strong preference for the production of 3-methylated quinolones with trans-Δ2-unsaturated alkyl chains. Here, we complete the description of the pathway and decipher the biochemical rationale for this preference. Our data suggest that highly efficient methylation of the intermediate 2-aminobenzoylacetate to 2-(2′-aminobenzoyl)propionate (2-ABP), combined with substrate preference of the final condensing enzyme HmqBC for 2-ABP and a 3-alkenoyl donor, is the major factor determining the product pattern. Surprisingly, 2-ABP appears to largely decompose to 4-hydroxy-3-methyl-2(1H)-quinolone, indicating an enzymatic bottleneck created by HmqBC. While the diversity of quinolone products acting as a multitarget antibiotic cocktail may be advantageous, key enzymes of the pathway nevertheless have evolved toward promoting the production of congeners that are active especially toward gram-positive bacteria and fungi and, moreover, resist C3-targeted detoxification.