Quorum Sensing in the Plant Pathogen Erwinia carotovora subsp. carotovora: The Role of expR_Ecc

Abstract

The production of the main virulence determinants of the plant pathogen Erwinia carotovora subsp. carotovora, the extracellular cell wall-degrading enzymes, is partly controlled by the diffusible signal molecule N-(3-oxohexanoyl)-L-homoserine lactone (OHHL). OHHL is synthesized by the product of the expI/carI gene. Linked to expI we found a gene encoding a putative transcriptional regulator of the LuxR-family. This gene, expR(Ecc), is transcribed convergently to the expI gene and the two open reading frames are partially overlapping. The ExpR(Ecc) protein showed extensive amino acid sequence similarity to the repressor EsaR from Pantoea stewartii subsp. stewartii (formerly Erwinia stewartii subsp. stewartii) and to the ExpR(Ech) protein of Erwinia chrysanthemi. Inactivation of the E. carotovora subsp. carotovora expR(Ecc) gene caused no decrease in virulence or production of virulence determinants in vitro. In contrast, there was a slight increase in the maceration capacity of the mutant strain. The effects of ExpR(Ecc) were probably mediated by changes in OHHL levels. Inactivation of expR(Ecc) resulted in increased OHHL levels during early logarithmic growth. In addition, overexpression of expR(Ecc) caused a clear decrease in the production of virulence determinants and part of this effect was likely to be caused by OHHL binding to ExpR(Ecc). ExpR(Ecc) did not appear to exhibit transcriptional regulation of expI, but the effect on OHHL was apparently due to other mechanisms.

Erwinia carotovoraSubspecies Produce Duplicate Variants of ExpR, LuxR Homologs That ActivatersmATranscription but Differ in Their Interactions withN-Acylhomoserine Lactone Signals

“…The absence of a signal with SCRI193 chromosomal DNA in our assay could have been due to a low level of homology between expR2 of Ecc71 and the corresponding gene in SCRI193. We note that expR1 has been identified previously (1,7) in some of the strains used by Burr et al After taking into consideration all of these observations, we concluded that the QS system of E. carotovora subspecies consists of ExpR1, ExpR2, and AhlI, the AHL synthase. As shown in Fig.…”

“…Previous studies demonstrated that mutation of ExpR1 in an AhlI ϩ background had no effect on the levels of RsmA and exoenzymes (1,12). We attribute the lack of effects of ExpR1 or ExpR2 in the AhlI ϩ strain to neutralization of ExpR variants by activities of AHL analogs.…”

“…The ability of these bacteria to cause soft rot disease in many plants and plant products depends mostly on the quantity and quality of the exoenzymes produced. Indeed, recent studies have disclosed that exoenzyme production is elaborately regulated in these bacteria by various transcriptional factors, posttranscriptional regulators, plant signals, and quorum-sensing (QS) signals (N-acylhomoserine lactones [AHLs]) (1,8,11,15,17,18,19,21,28,29,33,34,38,44). Two of these factors, regulatory components designated the RsmA-RsmB RNA pair (see below) and AHL, play a critical central role in exoenzyme production.…”

“…In E. carotovora subsp. carotovora, the AHL synthase gene, expI (ahlI), and an AHL receptor gene, expR1 (ahlR), overlap and are convergently transcribed (1,39,48). However, ExpR1 was found not to control ahlI expression or affect AHL production.…”

Quorum Sensing in Nitrogen-Fixing Rhizobia

“…Once bound, LuxR facilitates the binding of RNA polymerase to the target promoter (157)(158)(159), leading to activation of transcription. Although most instances show AHLbound LuxR-type proteins to function as transcriptional activators, a few examples of these proteins have been reported to function as repressors (2,13,15).…”