The complex quorum sensing circuitry of Burkholderia thailandensis is both hierarchically and homeostatically organized

Abstract

The genome of the bacterium Burkholderia thailandensis encodes for three complete LuxI/LuxR-type quorum sensing (QS) systems: BtaI1/BtaR1 (QS-1), BtaI2/BtaR2 (QS-2), and BtaI3/BtaR3 (QS-3). The LuxR-type transcriptional regulators BtaR1, BtaR2, and BtaR3 modulate the expression of target genes in association with various N-acyl-L-homoserine lactones (AHLs) as signaling molecules produced by the LuxI-type synthases BtaI1, BtaI2, and BtaI3. We have systematically dissected the complex QS circuitry of B. thailandensis strain E264. Direct quantification of octanoyl-homoserine lactone (C8-HSL), N-3-hydroxy-decanoyl-homoserine lactone (3OHC10-HSL), and N-3-hydroxy-octanoyl-homoserine lactone (3OHC8-HSL), the primary AHLs produced by this bacterium, was performed in the wild-type strain and in QS deletion mutants. This was compared to the expression of btaI1, btaI2, and btaI3 using chromosomal mini-CTX-lux transcriptional reporters. Furthermore, transcription of btaR1, btaR2, and btaR3 was monitored by quantitative reverse-transcription PCR (qRT-PCR). We observed that C8-HSL, 3OHC10-HSL, and 3OHC8-HSL are differentially produced over time during bacterial growth and correlate with the btaI1, btaI2, and btaI3 genes expression profiles, revealing a sequential activation of the corresponding QS systems. Moreover, transcription of the btaR1, btaR2, and btaR3 genes is modulated by AHLs, showing that their regulation depend on themselves, and on other systems. We conclude that the three QS systems in B. thailandensis are interdependent, suggesting that they cooperate dynamically and function in a concerted manner in modulating the expression of QS target genes through a sequential regulatory network.\n\nImportanceQuorum sensing (QS) is a widespread bacterial communication system coordinating the expression of specific genes in a cell density-dependent manner and allowing bacteria to synchronize their activities and to function as multicellular communities. QS plays a crucial role in bacterial pathogenicity by regulating the expression of a wide spectrum of virulence/survival factors and is essential to environmental adaptation. The results presented here demonstrate that the multiple QS systems coexisting in the bacterium Burkholderia thailandensis, considered as the avirulent version of the human pathogen Burkholderia pseudomallei and thus commonly used as an alternative study model, are hierarchically and homeostatically organized. We found these QS systems finely integrated into a complex regulatory network, including transcriptional and post-transcriptional interactions, and further incorporating growth stages and temporal expression. These results provide a unique, comprehensive illustration of a sophisticated QS network and will contribute to a better comprehension of the regulatory mechanisms that can be involved in the expression of QS-controlled genes, in particular those associated with the establishment of host-pathogen interactions and acclimatization to the environment.