Altering substrate chain length specificity of an acylhomoserine lactone synthase in bacterial communication.

Quorum sensing mediated by specific signal compounds (autoinducers) allows bacteria to monitor their cell density and enables a synchronized regulation of target gene sets. The best studied group of autoinducers are the acylhomoserine lactones (AHSLs), which are central to the regulation of virulence in many plant and animal pathogens. Variation of the acyl side chain of the AHSLs underlies the observed species specificity of this communication system. Here we show that even different strains of the plant pathogen Erwinia carotovora employ different dialects of this language and demonstrate the molecular basis for the acyl chain length specificity of distinct AHSL synthases. Under physiological concentrations, only the cognate AHSL with the "right" acyl chain is recognized as a signal that will switch on virulence genes. Mutagenesis of the AHSL synthase gene expI(SCC1) identified the changes M127T and F69L as sufficient to effectively alter ExpI(SCC1) (an N-3-oxohexanoyl-l-homoserine lactone producer) substrate specificity to that of an N-3-oxooctanoyl-l-homoserine lactone producer. Our data identify critical residues that define the size of the substrate-binding pocket of the AHSL synthase and will help in understanding and manipulating this bacterial language.