Hierarchical transcriptional regulation of quorum-sensing genes inVibrio harveyi

Abstract

17In vibrios, quorum sensing controls hundreds of genes that are required for cell density-specific 18 behaviors including bioluminescence, biofilm formation, competence, secretion, and swarming 19 motility. The central transcription factor in the quorum-sensing pathway is LuxR/HapR, which 20 directly regulates ~100 genes in the >400-gene regulon of Vibrio harveyi. Among these directly 21 controlled genes are 15 transcription factors, which we predicted would comprise the second 22 tier in the hierarchy of the quorum-sensing regulon. We confirmed that LuxR binds to the 23 promoters of these genes in vitro and quantified the extent of LuxR activation or repression of 24 transcript levels. RNA-seq indicates that most of these transcriptional regulators control only a 25 few genes, with the exception of MetJ, which is a global regulator. The genes regulated by 26 these transcription factors are predicted to be involved in methionine and thiamine biosynthesis, 27 membrane stability, RNA processing, c-di-GMP degradation, sugar transport, and other cellular 28 processes. These data support a hierarchical model in which LuxR directly regulates 15 29 transcription factors that drive the second level of the gene expression cascade to influence cell 30 density-dependent metabolic states and behaviors in V. harveyi. 31 32 33 Importance 34 Quorum sensing is important for survival of bacteria in nature and influences the actions of 35 bacterial groups. In the relatively few studied examples of quorum sensing-controlled genes, 36 these genes are associated with competition or cooperation in complex microbial communities 37 and/or virulence in a host. However, quorum sensing in vibrios controls the expression of 38 hundreds of genes, and their functions are mostly unknown or uncharacterized. In this study, we 39 identify the regulators of the second-tier of gene expression in the quorum-sensing system of 40 the aquatic pathogen Vibrio harveyi. Our identification of regulatory networks and metabolic 41 pathways controlled by quorum sensing can be extended and compared to other Vibrio species 42 to understand the physiology, ecology, and pathogenesis of these organisms. 43 44 45 Introduction 46 47 Bacteria coordinate gene expression in response to an array of external stimuli including 48 nutrients, pH, and temperature. Other environmental cues that control gene expression are 49 autoinducers (AIs) -signaling molecules that are produced by bacterial communication systems 50 termed quorum sensing (QS) (1). In nature, bacteria live in complex microbial communities 51 where population-wide synchronization is critical for survival. While QS circuitry differs between 52 organisms, the core function remains constant -to enable locally unified gene expression. The 53 QS system of Vibrio harveyi, a significant marine pathogen, has been studied for decades and 54 has yielded a wealth of knowledge about communication in Gram-negative bacteria (2-4). In this 55 bacterium, three distinct AIs are produced intracellularly and diffuse into the ...