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, over-expression 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.