Capsular polysaccharide biosynthesis and pathogenicity in Erwinia stewartii require induction by an N-acylhomoserine lactone autoinducer

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Abstract
N-Acylhomoserine lactone (acyl-HSL)-mediated gene expression, also called autoinduction, is conserved among diverse gram-negative bacteria. In the paradigm Vibrio fischeri system, bioluminescence is autoinducible, and the lux operon requires the transcriptional activator LuxR and the acyl-HSL autoinducer for expression. The production of the acyl-HSL signal molecule is conferred by the luxI gene, and luxR encodes the transcriptional regulator. We show here that Erwinia stewartii, the etiological agent of Stewart's wilt of sweet corn, synthesizes an acyl-HSL. Mass spectral analysis identified the signal molecule as N-(-3-oxohexanoyl)-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 indicates that EsaI and EsaR are homologs of LuxI and LuxR, respectively. EsaR can repress its own expression but seems not to regulate the expression of esaI. The untranslated 5' region of esaR contains an inverted repeat with similarity to the lux box-like elements located in the promoter regions of other gene systems regulated by autoinduction. However, unlike the other systems, in which the inverted repeats are located upstream of the -35 promoter elements, the esaR-associated repeat overlaps a putative -10 element. We mutagenized the esaI gene in E. stewartii by gene replacement. The mutant no longer produced detectable levels of the acyl-HSL signal, leading to a concomitant loss of extracellular polysaccharide capsule production and pathogenicity. Both phenotypes were restored by complementation with esal or by exogenous addition of the acyl-HSL.