A combination of luxR1 and luxR2 genes activates Pr-promoters of psychrophilic Aliivibrio logei lux-operon independently of chaperonin GroEL/ES and protease Lon at high concentrations of autoinducer

The atypical organization of the luxI/R family genes in AHL-driven quorum-sensing circuits

Quorum sensing (QS) is an elaborate regulatory mechanism associated with virulence and bacterial adaptation to the changing environment. QS is widespread in Proteobacteria and acts primarily through N-acylhomoserine lactone (AHL) signals. At the core of the AHL-driven QS systems are the AHL synthase gene (luxI family) and its cognate transcriptional regulator gene (luxR family). Several QS systems display one or more genes intervening between the luxI and luxR, in which gene arrangements are notably different due to the relative position and the transcriptional orientation between the essential luxI/R and the genes of location correlation. These adjacent genes may exert a regulatory impact on the primary QS genes or contribute toward an extension of QS regulatory control. In this review, we describe the organization of AHL-driven QS genes based on previous research and specific genome databases and provide new insights into these atypical QS gene arrangements.

LitR directly upregulates autoinducer synthesis and luminescence in Aliivibrio logei

LitR is a master-regulator of transcription in the ainS/R and luxS/PQ quorum sensing (QS) systems of bacteria from Vibrio and Aliivibrio genera. Here, we for the first time directly investigated the influence of LitR on gene expression in the luxI/R QS system of psychrophilic bacteria Aliivibrio logei. Investigated promoters were fused with Photorhabdus luminescens luxCDABE reporter genes cassette in a heterological system of Escherichia coli cells, litR A. logei was introduced into the cells under control of P_lac promoter. LitR has been shown to upregulate genes of autoinducer synthase (luxI), luciferase and reductase (luxCDABE), and this effect doesn’t depend on presence of luxR gene. To a much lesser degree, LitR induces luxR1, but not the luxR2 — the main luxI/R regulator. Enhanced litR expression leads to an increase in a LuxI-autoinducer synthesis and a subsequent LuxR-mediated activation of the luxI/R QS system. Effect of LitR on luxI transcription depends on lux-box sequence in luxI promoter even in absence of luxR (lux-box is binding site of LuxR). The last finding indicates a direct interaction of LitR with the promoter in the lux-box region. Investigation of the effect of LitR A. logei on luxI/R QS systems of mesophilic Aliivibrio fischeri and psychrophilic Aliivibrio salmonicida showed direct luxR-independent upregulation of luxI and luxCDABE genes. To a lesser degree, it induces luxR A. fischeri and luxR1 A. salmonicida. Therefore, we assume that the main role of LitR in cross-interaction of these three QS systems is stimulating the expression of luxI.

Influence of the luxR Regulatory Gene Dosage and Expression Level on the Sensitivity of the Whole-Cell Biosensor to Acyl-Homoserine Lactone

Aliivibrio fischeri LuxR and Aliivibrio logei LuxR1 and LuxR2 regulatory proteins are quorum sensing transcriptional (QS) activators, inducing promoters of luxICDABEG genes in the presence of an autoinducer (3-oxo-hexanoyl-l-homoserine lactone). In the Aliivibrio cells, luxR genes are regulated by HNS, CRP, LitR, etc. Here we investigated the role of the luxR expression level in LuxI/R QS system functionality and improved the whole-cell biosensor for autoinducer detection. Escherichia coli-based bacterial lux-biosensors were used, in which Photorhabdus luminescensluxCDABE genes were controlled by LuxR-dependent promoters and luxR, luxR1, or luxR2 regulatory genes. We varied either the dosage of the regulatory gene in the cells using additional plasmids, or the level of the regulatory gene expression using the lactose operon promoter. It was shown that an increase in expression level, as well as dosage of the regulatory gene in biosensor cells, leads to an increase in sensitivity (the threshold concentration of AI is reduced by one order of magnitude) and to a two to threefold reduction in response time. The best parameters were obtained for a biosensor with an increased dosage of luxRA. fischeri (sensitivity to 3-oxo-hexanoyl-l-homoserine lactone reached 30–100 pM).

Seasonal changes in luminescent intestinal microflora of the fish inhabiting the Bering and Okhotsk seas

Here, we present a study of luminescent intestinal microflora of the fish inhabiting Bering and Okhotsk seas in summer and winter seasons. Sampling of intestinal luminescent microflora was carried for several years, with all recovered species belonging to psychrophilic bacteria of either Aliivibrio logei or Photobacterium phosphoreum species. A seasonal change in fish intestinal luminescent microflora detected include an increase in prevalence of P. phosphoreum bacteria in summer and an increase in prevalence of A. logei bacteria in winter seasons. In fact, 90% of all luminescent bacteria isolated in winter period (January-March) were A. logei, while 88% of luminescent isolates recovered in summer period (July-September) were that of P. phosphoreum species. Seasonal changes were similar across all six sampling expeditions, three in winter and three in summer seasons, evenly spread through 2010-2018 period.

Comparative analysis of Aliivibrio logei luxR1 and luxR2 genes regulation in Escherichia coli cells

Regulation of Aliivibrio logei luxR1 and luxR2 genes was evaluated in Escherichia coli cells with use of transcriptional fusions of luxR1 and luxR2 promoter/operator regions with the Photorhabdus luminescens luxCDABE reporter gene cassette. Expression of the luxR1 and luxR2 genes was shown to largely depend on the CRP as activator. The hns::kan mutation increases the expression of luxR2 gene by two to three orders of magnitude and luxR1 gene by two to threefold. The LuxR1 and LuxR2 proteins in the presence of autoinducer (N-acyl homoserine lactone, AI) separately as well as together considerably enhanced the transcription of the luxR2 gene. In contrast, the transcription of luxR1 gene decreases depending on AI concentration in the presence of the luxR1 and luxR2 genes combination. It was identified that the promoter region of luxR2 gene consists of two promoters: P_crp is located downstream of the crp box and P_lux-box is located between the crp box and the lux box.