Structure-Guided Biochemical Analysis of Quorum Signal Synthase Specificities

Many bacteria use membrane-diffusible small molecule quorum signals to coordinate gene transcription in response to changes in cell density, known as quorum sensing (QS). Among these, acyl-homoserine lactones (AHL) are widely distributed in Proteobacteria and are involved in controlling the expression of virulence genes and biofilm formation in pathogens, such as Pseudomonas aeruginosa. AHL molecules are specifically biosynthesized by the cognate LuxI type AHL synthases using S-adenosylmethionine (SAM) and either acyl carrier protein (ACP)- or CoA-coupled fatty acids through a two-step reaction. Here, we characterize a CoA-dependent LuxI synthase from Rhodopseudomonas palustris that utilizes an aryl-CoA substrate that is environmentally derived, specifically p-coumaric acid. We leverage structures of this aryl-CoA-dependent synthase, along with our prior studies of an acyl-CoA-dependent synthase, to identify residues that confer substrate chain specificity in these enzymes. We test our predictions by carrying out biochemical, kinetic, and structural characterization of representative AHL signal synthases. Our studies provide an understanding of various AHL synthases that may be deployed in synthetic biological applications and inform on the design of specific small molecule therapeutics that can restrict virulence by targeting quorum signaling.