hierarchical quorum-sensing system in Yersinia pseudotuberculosis is involved in the regulation of motility and clumping

In cell‐free Yersinia pseudotuberculosis culture supernatants, we have chemically characterized three N‐acyl homoserine lactone (AHL) molecules, N‐octanoyl homoserine lactone (C8‐HSL), N‐(3‐oxohexanoyl)homoserine lactone (3‐oxo‐C6‐HSL) and N‐hexanoyl homoserine lactone (C6‐HSL). We have identified, cloned and sequenced two pairs of LuxR/I homologues termed YpsR/I and YtbR/I. In Escherichia coli at 37°C, YpsI and YtbI both synthesize C6‐HSL, although YpsI is responsible for 3‐oxo‐C6‐HSL and YtbI for C8‐HSL synthesis respectively. However, in a Y. pseudotuberculosis ypsI‐negative background, YtbI appears capable of adjusting the AHL profile from all three AHLs at 37°C and 22°C to the absence of 3‐oxo‐C6‐HSL at 28°C. Insertion deletion mutagenesis of ypsR leads to the loss of C8‐HSL at 22°C, which suggests that at this temperature the YpsR protein is involved in the hierarchical regulation of the ytbR/I locus. When compared with the parent strain, the ypsR and ypsI mutants exhibit a number of phenotypes, including clumping (ypsR mutant), overexpression of a major flagellin subunit (ypsR mutant) and increased motility (both ypsR and ypsI mutants). The clumping and motility phenotypes are both temperature dependent. These data are consistent with a hierarchical quorum‐sensing cascade in Y. pseudotuberculosis that is involved in the regulation of clumping and motility.