The Type II Secretion Pathway in Vibrio cholerae Is Characterized by Growth Phase-Dependent Expression of Exoprotein Genes and Is Positively Regulated by σE

Vibrio cholerae , an etiological agent of cholera, circulates between aquatic reservoirs and the human gastrointestinal tract. The type II secretion (T2S) system plays a pivotal role in both stages of the lifestyle by exporting multiple proteins, including cholera toxin. Here, we studied the kinetics of expression of genes encoding the T2S system and its cargo proteins. We have found that under laboratory growth conditions, the T2S complex was continuously expressed throughout V. cholerae growth, whereas there was growth phase-dependent transcriptional activity of genes encoding different cargo proteins. Moreover, exposure of V. cholerae to different environmental cues encountered by the bacterium in its life cycle induced transcriptional expression of T2S. Subsequent screening of a V. cholerae genomic library suggested that σ E stress response, phosphate metabolism, and the second messenger 3′,5′-cyclic diguanylic acid (c-di-GMP) are involved in regulating transcriptional expression of T2S. Focusing on σ E , we discovered that the upstream region of the T2S operon possesses both the consensus σ E and σ 70 signatures, and deletion of the σ E binding sequence prevented transcriptional activation of T2S by RpoE. Ectopic overexpression of σ E stimulated transcription of T2S in wild-type and isogenic Δ rpoE strains of V. cholerae , providing additional support for the idea that the T2S complex belongs to the σ E regulon. Together, our results suggest that the T2S pathway is characterized by the growth phase-dependent expression of genes encoding cargo proteins and requires a multifactorial regulatory network to ensure appropriate kinetics of the secretory traffic and the fitness of V. cholerae in different ecological niches.