Groups of pathogenic bacteria employ diffusible signals to regulate their virulence in a concerted manner. Pseudomonas aeruginosa uses 4-hydroxy-2-alkylquinolines (HAQs), including HHQ and PQS, as unique signals. We demonstrate that octanoic acid is directly incorporated into HHQ. This finding rules out the long-standing hypothesis that 3-ketofatty acids are the precursors of HAQs. We found that HAQ biosynthesis, which requires the PqsABCD enzymes, proceeds by a two-step pathway: [1] PqsD mediates the synthesis of 2-aminobenzoylacetate (2-ABA) from anthraniloyl-CoA and malonyl-CoA, then [2] the decarboxylating coupling of 2-ABA to an octanoate group linked to PqsC produces HHQ, the direct precursor of PQS (Pseudomonas Quinolone Signal). PqsB is tightly associated with PqsC and required for the second step. This finding uncovers promising targets for the development of specific antivirulence drugs to combat this opportunistic pathogen.

The 3‐(3‐hydroxyalkanoyloxy)alkanoate (HAA) synthase RhlA is an essential enzyme involved in the biosynthesis of HAAs in Pseudomonas and Burkholderia species. RhlA modulates the aliphatic chain length in rhamnolipids, conferring distinct physicochemical properties to these biosurfactants exhibiting promising industrial and pharmaceutical value. A detailed molecular understanding of substrate specificity and catalytic performance in RhlA could offer protein engineering tools to develop designer variants involved in the synthesis of novel rhamnolipid mixtures for tailored eco‐friendly products. However, current directed evolution progress remains limited due to the absence of high‐throughput screening methodologies and lack of an experimentally resolved RhlA structure. In the present work, we used comparative modeling and chimeric‐based approaches to perform a comprehensive semi‐rational mutagenesis of RhlA from Pseudomonas aeruginosa. Our extensive RhlA mutational variants and chimeric hybrids between the Pseudomonas and Burkholderia homologs illustrate selective modulation of rhamnolipid alkyl chain length in both Pseudomonas aeruginosa and Burkholderia glumae. Our results also demonstrate the implication of a putative cap‐domain motif that covers the catalytic site of the enzyme and provides substrate specificity to RhlA. This semi‐rational mutant‐based survey reveals promising ‘hot‐spots’ for the modulation of RL congener patterns and potential control of enzyme activity, in addition to uncovering residue positions that modulate substrate selectivity between the Pseudomonas and Burkholderia functional homologs. Database Model data are available in the PMDB database under the accession number http://www.ncbi.nlm.nih.gov/protein/PM0081867.

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