IN-VIVO EVIDENCE THAT S-ADENOSYLMETHIONINE AND FATTY-ACID SYNTHESIS INTERMEDIATES ARE THE SUBSTRATES FOR THE LUXI FAMILY OF AUTOINDUCER SYNTHASES

Many gram-negative bacteria synthesize N-acyl homoserine lactone autoi nducer molecules as quorum-sensing signals which act as cell density-d ependent regulators of gene expression. We have investigated the in vi vo source of the acyl chain and homoserine lactone components of the a utoinducer synthesized by the LuxI homolog, TraI. In Escherichia coli, synthesis of N-(3-oxooctanoyl) homoserine lactone by TraI was unaffec ted in a fadD mutant blocked in beta-oxidative fatty acid degradation. Also, conditions known to induce the fad regulon did not increase aut oinducer synthesis. In contrast, cerulenin and diazoborine, specific i nhibitors of fatty acid synthesis, both blocked autoinducer synthesis even in a strain dependent on beta-oxidative fatty acid degradation fa r growth, These data provide the first in vivo evidence that the acyl chains in autoinducers synthesized by LuxI-family synthases are derive d from acyl-acyl carrier protein substrates rather than acyI coenzyme A substrates. Also, me show that decreased levels of intracellular S-a denosylmethionine caused by expression of bacteriophage T3 S-adenosylm ethionine hydrolase result in a marked reduction in autoinducer synthe sis, thus providing direct in vivo evidence that the homoserine lacton e ring of LuxI-family autoinducers is derived from S-adenosylmethionin e.