CAPSULAR POLYSACCHARIDE BIOSYNTHESIS AND PATHOGENICITY IN ERWINIA-STEWARTII REQUIRE INDUCTION BY AN N-ACYLHOMOSERINE LACTONE AUTOINDUCER

N-Acylhomoserine lactone (acyl-HSL)-mediated gene expression, also cal led autoinduction, is conserved among diverse gram-negative bacteria. In the paradigm Vibrio fischeri system, bioluminescence is autoinducib le, and the lux operon requires the transcriptional activator LuxR and the acyl-HSL autoinducer for expression. The production of the acyl-H SL signal molecule is conferred by the luxI gene, and luxR encodes the transcriptional regulator. We show here that Erwinia stewartii, the e tiological agent of Stewart's, wilt of sweet corn, synthesizes an acyl -HSL. Mass spectral analysis identified the signal molecule as N-(3-ox ohexanoyl)-L-homoserine lactone, which is identical to the V. fischeri autoinducer. We have cloned and sequenced the gene that confers acyl- HSL biosynthesis, called esaI, and the linked gene, esaR, that encodes a gene regulator. The two genes are convergently transcribed and show an unusual overlap of 31 bp at their 3' ends. Sequence analysis indic ates that EsaI and EsaR are homologs of LuxI and LuxR, respectively. E saR can repress its own expression but seems not to regulate the expre ssion of esaI. The untranslated 5' region of esaR contains an inverted repeat with similarity to the lux box-like elements located in the pr omoter regions of other gene systems regulated by autoinduction. Howev er, unlike the other systems, in which the inverted repeats are locate d upstream of the -35 promoter elements, the esaR-associated repeat ov erlaps a putative -10 element. We mutagenized the esaI gene in E. stew artii by gene replacement. The mutant no longer produced detectable le vels of the acyl-HSL signal, leading to a concomitant loss of extracel lular polysaccharide capsule production and pathogenicity. Both phenot ypes were restored by complementation with esaI or by exogenous additi on of the acyl-HSL.