GENERATION OF CELL-TO-CELL SIGNALS IN QUORUM SENSING - ACYL HOMOSERINE LACTONE SYNTHASE ACTIVITY OF A PURIFIED VIBRIO-FESCHERI LUXI PROTEIN

Many bacteria use acyl homoserine lactone signals to monitor cell dens ity in a type of gene regulation termed quorum sensing and response, S ynthesis of these signals is directed by homologs of the luxI gene of Vibrio fischeri, This communication resolves two critical issues conce rning the synthesis of the V. fischeri signal. (i) The luxI product is directly involved in signal synthesis-the protein is an acyl homoseri ne lactone synthase; and (ii) the substrates for acyl homoserine lacto ne synthesis are not amino acids from biosynthetic pathways or fatty a cid degradation products, but rather they are S-adenosylmethionine (SA M) and an acylated acyl carrier protein (ACP) from the fatty acid bios ynthesis pathway, We purified a maltose binding protein-LuxI fusion po lypeptide and showed that, when provided with the appropriate substrat es, it catalyzes the synthesis of an acyl homoserine lactone, In V. fi scheri, luxI directs the synthesis of N-(3-oxohexanoyl)homoserine lact one and hexanoyl homoserine lactone. The purified maltose binding prot ein-LuxI fusion protein catalyzes the synthesis of hexanoyl homoserine lactone from hexanoyl-ACP and SAM, There is a high level of specifici ty for hexanoyl-ACP over ACPs with differing acyl group lengths, and h exanoyl homoserine lactone was not synthesized when SAM was replaced w ith other amino acids, such as methionine, S-adenosylhomocysteine, hom oserine, or homoserine lactone, or when hexanoyl-SAM was provided as t he substrate. This provides direct evidence that the LuxI protein is a n autoinducer synthase that catalyzes the formation of an amide bond b etween SAM and a fatty acyl-ACP and then catalyzes the formation of th e acyl homoserine lactone from the acyl-SAM intermediate.