Identification and Characterization of Four LuxR-Controlled Transcriptional Regulators in the Vibrio harveyi Quorum-Sensing Circuit

Abstract

Bacteria monitor the number of cells in their environment by producing, detecting, and responding to extracellular signaling molecules called autoinducers (AIs). This communication process, called quorum sensing, allows populations of bacteria to collectively control expression of hundreds of genes, including those involved in bioluminescence, virulence factor production, and biofilm formation, in response to changes in cell density. In Vibrio harveyi, LuxR acts as the master high cell density (HCD) quorum-sensing regulator of approximately 625 genes, including genes encoding fourteen putative transcription factors. While significant progress has been made to understand how LuxR is regulated, the roles of downstream transcription factors are unknown. In this study, we identified four LuxR-controlled transcription factors in the quorum-sensing circuit. We investigate regulation of four transcriptional regulators VIBHAR_02610, 02611, 06181, and 06912 during quorum sensing. VIBHAR_02610, 02611, and 06912 are repressed by LuxR. We show that VIBHAR_02610, 02611, and 06912 are activated at low cell density (LCD), which could allow these regulators to work along with AphA, the LCD master regulator, to control the LCD transcriptional program. In fact, the LuxR-repressed transcriptional regulators appear to exert feedback on other regulators in the quorumsensing circuit. Additionally, we show that VIBHAR_06181, which is homologous to the betI repressor of the Escherichia coli osmotic-stress response, is activated by LuxR. In turn, VIBHAR_06181 alters expression of aphA and one of the quorum regulatory RNAs at LCD and in response to changes in osmolality. Thus, VIBHAR_06181 may play a role altering quorum sensing in response to osmotic stress in V. harveyi.