Quorum Sensing Gene Regulation by LuxR/HapR Master Regulators in Vibrios

Abstract

The coordination of group behaviors in bacteria is accomplished via the cell-cell signaling process called quorum sensing. Vibrios have historically been models for studying bacterial communication due to the diverse and remarkable behaviors controlled by quorum sensing in these bacteria, including bioluminescence, type III and type VI secretion, biofilm formation, and motility. Here, we discuss the Vibrio LuxR/HapR family of proteins, the master global transcription factors that direct downstream gene expression in response to changes in cell density. These proteins are structurally similar to TetR transcription factors but exhibit distinct biochemical and genetic features from TetR that determine their regulatory influence on the quorum sensing gene network. We review here the gene groups regulated by LuxR/HapR and quorum sensing and explore the targets that are common and unique among Vibrio species.

Keywords: HapR; LuxR; SmcR; Vibrio; Vibrio cholerae; Vibrio harveyi; gene regulation; quorum sensing.

FIG 1

Model of the V. harveyi quorum sensing system. The autoinducer synthases LuxM, CqsS, and LuxS produce AI-1, CAI-1, and AI-2, respectively. At LCD, autoinducers are at low concentrations in the external environment. LuxO∼P activates qrr gene expression. The Qrr sRNAs activate the expression of AphA and repress the expression of LuxR, LuxM, and LuxO. High levels of AphA and low levels of LuxR together regulate individual behavior genes. AphA autorepresses its expression and feeds back to repress the expression of the qrr genes and luxR. LuxR autorepresses its expression, represses aphA, and activates the qrr genes. At HCD, autoinducer concentrations are high, and AI-1, CAI-1, and AI-2 bind to the LuxN, CqsS, and LuxPQ receptors, respectively. The receptors dephosphorylate LuxU. Thus, LuxU does not phosphorylate LuxO, and the Qrrs are not expressed. LuxR is expressed at high levels, and AphA is not expressed. LuxR regulates group behavior genes. LuxR autorepresses its own expression and feeds back to repress aphA transcription and to activate qrr transcription.

FIG 2

LuxR/HapR homolog structural and DNA binding properties. (A) Superimposed crystal structures of the TetR proteins QacR, HapR, and SmcR (32, 33, 84). Structures were superimposed with DaliLite (142) and the figures created with PyMOL. The DNA-bound QacR dimer superimposed with HapR or SmcR both resulted in a root mean square deviation (RMSD) of 3.1. Superimposition of HapR to SmcR results in an RMSD of 1.5. HapR is shown in cyan (PDB: 2PBX), SmcR is shown in light blue (PDB: 3KZ9), and for the QacR-DNA structure, QacR is shown in dark blue and the DNA in gray (PDB: 1JT0) (32, 33, 84). (B) LuxR DNA binding motifs from ChIP-seq data grouped by the location of the binding site peak. Top, all LuxR DNA binding peaks from ChIP-seq; middle, peaks in promoters of repressed genes; bottom, peaks in promoters of activated genes. The arrows indicate dyad symmetry in the binding site. Image reproduced with permission (74).

FIG 3

Overlapping regulons of Vibrio LuxR-type regulators. The LuxR, HapR, SmcR, and OpaR regulons, which were previously published (77, 78, 80, 82), were analyzed to identify mutual gene constituents via pairwise reciprocal BLAST searches (143). The numbers of homologous genes identified between pairs of regulons are shown in circles and connected by arrows. Genes associated with major cellular processes are listed in the brackets extending from each circled number. Lists are not comprehensive; the paired lists contain genes encoding hypothetical proteins with no annotated function. Vibrio parahae., Vibrio parahaemolyticus.