H-NS: an overarching regulator of the Vibrio cholerae life cycle

Abstract

Vibrio cholerae has become a model organism for studies connecting virulence, pathogen evolution and infectious disease ecology. The coordinate expression of motility, virulence and biofilm enhances its pathogenicity, environmental fitness and fecal-oral transmission. The histone-like nucleoid structuring protein negatively regulates gene expression at multiple phases of the V. cholerae life cycle. Here we discuss: (i) the regulatory and structural implications of H-NS chromatin-binding in the two-chromosome cholera bacterium; (ii) the factors that counteract H-NS repression; and (iii) a model for the regulation of the V. cholerae life cycle that integrates H-NS repression, cyclic diguanylic acid signaling and the general stress response.

Keywords: H-NS; Life cycle; Vibrio cholerae.

Fig. 1. Comparison of the distribution of H-NS clusters in the V. cholerae and E. coli genomes

A. H-NS occupancy along V. cholerae chromosomes I (left) and II (right) is represented by blue bars. Top blue bars correspond to ChIP-Seq peaks in V. cholerae C7258 [19,20]. Bottom blue bars correspond to ChIP-Seq peaks in V. cholerae C6706 [31]. B. H-NS occupancy along the chromosome of E. coli K-12 strain MG1655 is represented by red bars [30]. Deviation from average GC content is plotted along chromosomes in black.

Fig. 2. Transcriptional silencing by H-NS and anti-silencing of the ToxR regulon

H-NS repression is represented by a red line through the intergenic regions. The blue arrows indicate transcription activation at the corresponding promoter. Abbreviations: CT, cholera holotoxin; IHF, integration host factor.

Fig. 3. Model for the regulation of vps and rbm gene expression by H-NS transcriptional silencing and anti-silencing

H-NS binds to the promoters of vps and rbm genes to silence their expression when the c-di-GMP concentration is low. When the c-di-GMP pool is elevated, VpsR and VpsT sequentially displace H-NS from promoters to turn on gene expression. Diminished biosynthesis of the biofilm matrix at low c-di-GMP due to transcriptional silencing is indicated by a light gray font.

Fig. 4. Regulatory pathway controlling RpoS-dependent activation of motility and detachment

In a low-cell-density community, the c-di-GMP content is elevated, resulting in expression and activation of VpsR and VpsT. Motility is diminished by binding of H-NS to the flrA promoter and inhibition of FlrA by c-di-GMP. In a high-cell-density community, HapR is expressed to lower the c-di-GMP pool and terminate transcription of vpsT. Environmental stress and the absence of VpsT favor expression of RpoS. RpoS further diminishes the c-di-GMP pool to enhance the activity of FlrA and activates expression of IHF that lessens H-NS occupancy at the flrA promoter. In addition, initiation of flrA transcription by σ^S is more resistant to H-NS remaining bound to the promoter. Downregulated or inactive transcription factors, as well as disabled regulatory conections, are represented in light graey font. Symbols: ↓, positive regulation; ⊥, negative regulation.