Role of the Histone-Like Nucleoid Structuring Protein in Colonization, Motility, and Bile-Dependent Repression of Virulence Gene Expression in Vibrio cholerae

Abstract

Bile-mediated repression of virulence gene expression is relieved in a Vibrio cholerae hns mutant. The mutant also exhibited reduced motility due to lower flrA expression, higher in vivo production of the virulence factors, and lower colonization efficiency. The colonization defect of the mutant was due to low FlrA production.Vibrio cholerae, a gram-negative, noninvasive enteric bacterium is the causative agent of the diarrheal disease cholera (13). Cholera continues to be a cause of human mortality, especially in the developing countries, where conditions of poor sanitation, lack of safe drinking water, malnourishment, war, and famine contribute to regular episodes of cholera, estimated to be more than 5 million cases per year.Expression of a subset of virulence factors of V. cholerae is controlled by a regulatory cascade known as the ToxR regulon (14). The transcriptional regulators ToxR and TcpP (8) act synergistically to control the production of a second regulator, ToxT (5), that directly activates the expression of several virulence genes, including ctxAB and tcpA, coding for cholera toxin (CT) and TcpA, the major subunit of the toxin-coregulated pilus, respectively. The toxin-coregulated pilus is thought to be essential for colonization of the intestinal epithelium by the bacterium (10, 27). Expression of the ToxR regulon is strongly influenced by physicochemical parameters that exert their effects at different levels of the regulatory cascade (reviewed in references 17, 20, and 26). Under nonpermissive conditions of temperature and pH, transcription of the tcpP gene is repressed, leading to down regulation of the entire virulence regulon (15). However, under anaerobic or microaerobic conditions, TcpP, ToxR, and ToxT are produced and the tcpA gene is optimally expressed, but expression of ctxAB is drastically reduced (16,18). The presence of bile influences expression of the virulence regulon in yet another manner. Although production of the regulatory factors ToxR, TcpP, and ToxT is unaffected in cells grown in the presence of bile, expression of both ctxAB and tcpA is drastically reduced by an unknown mechanism (7, 24). A role of the nucleoid-associated histone-like nucleoid structuring protein (H-NS) in the silencing of virulence gene expression under nonpermissive conditions of temperature, pH, and oxygen concentration has been demonstrated (22,28). In this study we have shown that H-NS also represses ctxAB and tcpA gene expression in cells grown in the presence of bile and in vivo in rabbit intestine. Furthermore, the motility and colonization efficiency of a V. cholerae hns mutant strain were lower than those of the wild-type strain.A V. cholerae hns mutant produces CT and TcpA in the presence of bile. The V. cholerae O395 hns mutant strain O395H29, previously used to demonstrate the role of H-NS in anaerobic repression of CT production (16), was used in this study. CT in culture supernatants and sonicated cell lysates of V. cholerae strains O395 and O395H29 (hns) grown under inducing conditions (LB...

Interaction of the Histone-Like Nucleoid Structuring Protein and the General Stress Response Regulator RpoS at Vibrio cholerae Promoters That Regulate Motility and Hemagglutinin/Protease Expression

“…Consistent with this role, H-NS has been shown to silence virulence gene expression in V. cholerae by acting at different levels of the ToxR regulatory cascade, which includes the toxT, tcpA, and ctxA promoters (37). Further, V. cholerae hns mutants have been reported to exhibit diminished motility and intestinal colonization capacity (18,27,49,53). There are numerous evidences indicating that repression by H-NS can be relieved in response to environmental stimuli that activate the expression of other regulators whose binding site overlaps with that of H-NS.…”

“…The general stress response regulator RpoS and the nucleoid structuring protein H-NS are global regulators known to affect intestinal colonization, virulence gene expression, and mucosal escape in the cholera bacterium (18,27,32,35,49). Here, we focus on the role of these proteins in the transcription of motility regulators rpoN and flrA as well as hapA, encoding HA/protease.…”

Anti-silencing: overcoming H-NS-mediated repression of transcription in Gram-negative enteric bacteria

“…Examples include H-NS-repressed virulence genes in pathogenic E. coli (Bustamante et al, 2001;Corbett et al, 2007;Haack et al, 2003;Laaberki et al, 2006;Müller et al, 2006;Torres et al, 2007), Erwinia spp. (Nasser & Reverchon, 2002), Proteus mirabilis (Coker et al, 2000), Shigella flexneri Prosseda et al, 2004), Vibrio cholerae (Ghosh et al, 2006;Nye et al, 2000) and Yersinia spp. (Cathelyn et al, 2007;Ellison & Miller, 2006b;Heroven et al, 2007).…”

Unique Virulence Properties of Yersinia enterocolitica O:3

“…The histone-like protein H-NS was shown to silence the inv promoter in a complex with YmoA, a member of the Hha regulator family (Ellison et al, 2004;Lawrenz & Miller, 2007). H-NS is necessary for the nucleoid packaging of the bacterial chromosome and regulates a variety of genes depending on temperature (Atlung & Ingmer, 1997;Ghosh et al, 2006;Müller et al, 2006). Gene silencing mediated by H-NS is caused by preferentially binding to AT-rich DNA sequences displaying intrinsic curvatures (Olivares-Zavaleta et al, 2006;Rimsky, 2004;Schröder & Wagner, 2002).…”