The Molecular Basis of a Nitric Oxide Responsive Quorum Sensing Circuit in Vibrio harveyi

Abstract

The discovery of bacteria's ability to coordinate population-wide undertakings has paved the way to understanding many biological processes once thought to be restricted to multi-cellular organisms. Bacteria use small molecules to communicate with one another and gauge the surroundings to formulate a unified response. This process, known as quorum sensing (QS), regulates diverse functions such as bioluminescence, biofilm formation, and virulence. Vibrio harveyi have three parallel QS circuits that regulate bioluminescence. Each system involves the synthesis of specific small molecule called autoinducer that binds to its cognate receptor histidine kinase and regulates phosphorylation. The three pathways converge and exchange phosphate with a common phospho-relay protein called LuxU. LuxU transfers phosphate to LuxO, a response regulator that controls expression of LuxR and, ultimately, the quorum sensing response. In this thesis project, we have identified nitric oxide (NO) as a signal molecule that participates in regulation of bioluminescence through the LuxU/LuxO/LuxR pathway. We show that V. harveyi display a NO concentration-dependent increase in light production that is regulated by expression of a NO-sensitive hnoX gene. We demonstrate that VIBHAR_01911, annotated as Heme-Nitric Oxide/Oxygen binding protein (H-NOX) binds NO at approximately picomolar level. NO-bound H-NOX interacts and regulates the phosphorylation of a histidine kinase named HqsK (H-NOX-associated quorum sensing kinase) that, like the other QS kinases, transfers phosphate to LuxU. Thus, NO concentration is factored into the quorum sensing output of V. harveyi via H-NOX-HqsK. This study reports the discovery of a fourth quorum sensing pathway in V. harveyi and suggests that bacteria use QS to integrate not only the density of bacteria, but also other diverse information, potentially even the presence of eukaryotes, into decisions about gene expression. Furthermore, this study provides characterization of a new pathway that could be targeted for the development of novel antibiotics.