Dual selection enhances the signaling specificity of a variant of the quorum-sensing transcriptional activator LuxR.

The transcription factor LuxR activates gene expression in response to binding the signaling molecule 3-oxo-hexanoyl-homoserine lactone (3OC6HSL), an acyl-HSL with a carbonyl substituent at the third carbon of the acyl chain. We previously described a LuxR variant, LuxR-G2E, that activates gene expression by binding a broader range of acyl-HSLs, including straight-chain acyl-HSLs to which LuxR does not respond. Here, we use a dual positive-negative selection system to identify a variant of LuxR-G2E that retains the response to straight-chain acyl-HSLs, but no longer responds to 3OC6HSL. A single mutation, R67M, reduces LuxR-G2E's response to acyl-HSLs having a carbonyl substituent at the third carbon of the acyl chain. This specificity-enhancing mutation would not have been identified by positive selection alone. The dual selection system provides a rapid and reliable method for identifying LuxR variants that have or lack the desired response to a given set of acyl-HSL signals. LuxR variants with altered signaling specificities might become useful components for constructing artificial cell-cell communication systems that program population level behaviors.