Characterization of diverse homoserine lactone synthases in Escherichia coli
(A) Synthase protein (S) catalyzes formation of the HSL species. Regulator protein (R) complexes with the HSL ligand and binds to the promoter, which induces expression of downstream genes. (B) Synthase proteins catalyze the production of HSL molecules by facilitating lactone ring formation from S-adenosyl methionine and attaching a variable R-group tail to the lactone ring from one of two donors, acyl-carrier proteins (ACP) or coenzyme A (CoA). (C) The cartoon illustrates an example of the consequence of crosstalk. Two pathways, SA > RA and SB > RB are designed to operate in parallel as a Boolean AND gate sensor. When each synthase generates a single predominant HSL, the output indicates the activity of both SA and SB as intended. However, if SA produces high levels of primary and secondary products that stimulate both RA and RB, a fault occurs where output is produced although the AND condition is not met.