Burkholderia thailandensis Methylated Hydroxyalkylquinolines: Biosynthesis and Antimicrobial Activity in Cocultures

Abstract

The bacterium <i>Burkholderia thailandensis</i> produces an arsenal of secondary metabolites that have diverse structures and roles in the ecology of this soil-dwelling bacterium. In coculture experiments, <i>B. thailandensis</i> strain E264 secretes an antimicrobial that nearly eliminates another soil bacterium, <i>Bacillus subtilis</i> strain 168. To identify the antimicrobial, we used a transposon mutagenesis approach. This screen identified antimicrobial-defective mutants with insertions in the <i>hmqA</i>, <i>hmqC</i>, and <i>hmqF</i> genes involved in biosynthesis of a family of 2-alkyl-4(1<i>H</i>)-quinolones called 4-hydroxy-3-methyl-2-alkenylquinolines (HMAQs), which are closely related to the <i>Pseudomonas aeruginosa</i> 4-hydroxy-2-alkylquinolines (HAQs). Insertions also occurred in the previously uncharacterized gene BTH_II1576 ("<i>hmqL</i>"). The results confirm that BTH_II1576 is involved in generating <i>N</i>-oxide derivatives of HMAQs (HMAQ-NOs). Synthetic HMAQ-NO is active against <i>B. subtilis</i> 168, showing ∼50-fold more activity than HMAQ. Both the methyl group and the length of the carbon side chain account for the high activity of HMAQ-NO. The results provide new information on the biosynthesis and activities of HMAQs and reveal new insight into how these molecules might be important for the ecology of <i>B. thailandensis</i><b>IMPORTANCE</b> The soil bacterium <i>Burkholderia thailandensis</i> produces 2-alkyl-4(1<i>H</i>)-quinolones that are mostly methylated 4-hydroxyalkenylquinolines, a family of relatively unstudied metabolites similar to molecules also synthesized by <i>Pseudomonas aeruginosa</i> Several of the methylated 4-hydroxyalkenylquinolines have antimicrobial activity against other species. We show that <i>Bacillus subtilis</i> strain 168 is particularly susceptible to <i>N</i>-oxidated methylalkenylquinolines (HMAQ-NOs). We confirmed that HMAQ-NO biosynthesis requires the previously unstudied protein HmqL. These results provide new information about the biology of 2-alkyl-4(1<i>H</i>)-quinolones, particularly the methylated 4-hydroxyalkenylquinolines, which are unique to <i>B. thailandensis</i> This study also has importance for understanding <i>B. thailandensis</i> secondary metabolites and has implications for potential therapeutic development.