A structural model for (GlcNAc)2 translocation via a periplasmic chitooligosaccharide-binding protein from marine Vibrio bacteria

Exploring the metabolic potential of Aeromonas to utilise the carbohydrate polymer chitin

Members of the Aeromonas genus are commonly found in natural aquatic ecosystems. However, they are also frequently present in non-chlorinated drinking water distribution systems. High densities of these bacteria indicate favorable conditions for microbial regrowth, which is considered undesirable. Studies have indicated that the presence of Aeromonas is associated with loose deposits and the presence of invertebrates, specifically Asellus aquaticus. Therefore, a potential source of energy in these nutrient poor environments is chitin, the structural shell component in these invertebrates. In this study, we demonstrate the ability of two Aeromonas strains, commonly encountered in drinking water distribution systems, to effectively degrade and utilize chitin as a sole carbon and nitrogen source. We conducted a quantitative proteomics study on the cell biomass and secretome from pure strain cultures when switching the nutrient source from glucose to chitin, uncovering a diverse array of hydrolytic enzymes and metabolic pathways specifically dedicated to the utilization of chitin. Additionally, a genomic analysis of different Aeromonas species suggests the general ability of this genus to degrade and utilize a variety of carbohydrate biopolymers. This study indicates the relation between the utilization of chitin by Aeromonas and their association with invertebrates such as A. aquaticus in loose deposits in drinking water distribution systems.

Two transmembrane transcriptional regulators coordinate to activate chitin-induced natural transformation in Vibrio cholerae

Transcriptional regulators are a broad class of proteins that alter gene expression in response to environmental stimuli. Transmembrane transcriptional regulators (TTRs) are a subset of transcriptional regulators in bacteria that can directly regulate gene expression while remaining anchored in the membrane. Whether this constraint impacts the ability of TTRs to bind their DNA targets remains unclear. Vibrio cholerae uses two TTRs, ChiS and TfoS, to activate horizontal gene transfer by natural transformation in response to chitin by inducing the tfoR promoter (PtfoR). While TfoS was previously shown to bind and regulate PtfoR directly, the role of ChiS in PtfoR activation remains unclear. Here, we show that ChiS directly binds PtfoR upstream of TfoS, and that ChiS directly interacts with TfoS. By independently disrupting ChiS-PtfoR and ChiS-TfoS interactions, we show that ChiS-PtfoR interactions play the dominant role in PtfoR activation. Correspondingly, we show that in the absence of ChiS, recruitment of the PtfoR locus to the membrane is sufficient for PtfoR activation when TfoS is expressed at native levels. Finally, we show that the overexpression of TfoS can bypass the need for ChiS for PtfoR activation. All together, these data suggest a model whereby ChiS both (1) recruits the PtfoR DNA locus to the membrane for TfoS and (2) directly interacts with TfoS, thereby recruiting it to the membrane-proximal promoter. This work furthers our understanding of the molecular mechanisms that drive chitin-induced responses in V. cholerae and more broadly highlights how the membrane-embedded localization of TTRs can impact their activity.

Two transmembrane transcriptional regulators coordinate to activate chitin-induced natural transformation in Vibrio cholerae

Transcriptional regulators are a broad class of proteins that alter gene expression in response to environmental stimuli. Transmembrane transcriptional regulators (TTRs) are a subset of transcriptional regulators in bacteria that can directly regulate gene expression while remaining anchored in the membrane. Whether this constraint impacts the ability of TTRs to bind their DNA targets remains unclear. Vibrio cholerae uses two TTRs, ChiS and TfoS, to activate horizontal gene transfer by natural transformation in response to chitin by inducing the tfoR promoter (P tfoR ). While TfoS was previously shown to bind and regulate P tfoR directly, the role of ChiS in P tfoR activation remains unclear. Here, we show that ChiS directly binds P tfoR upstream of TfoS, and that ChiS directly interacts with TfoS. By independently disrupting ChiS-P tfoR and ChiS-TfoS interactions, we show that ChiS-P tfoR interactions play the dominant role in P tfoR activation. Correspondingly, we show that in the absence of ChiS, recruitment of the P tfoR locus to the membrane is sufficient for P tfoR activation when TfoS is expressed at native levels. Finally, we show that the overexpression of TfoS can bypass the need for ChiS for P tfoR activation. All together, these data suggest a model whereby ChiS both (1) recruits the P tfoR DNA locus to the membrane for TfoS and (2) directly interacts with TfoS, thereby recruiting it to the membrane-proximal promoter. This work furthers our understanding of the molecular mechanisms that drive chitin-induced responses in V. cholerae and more broadly highlights how the membrane-embedded localization of TTRs can impact their activity.

AUTHOR SUMMARY

Living organisms inhabit diverse environments where they encounter a wide range of stressors. To survive, they must rapidly sense and respond to their surroundings. One universally conserved mechanism to respond to stimuli is via the action of DNA-binding transcriptional regulators. In bacterial species, these regulators are canonically cytoplasmic proteins that freely diffuse within the cytoplasm. In contrast, an emerging class of transmembrane transcriptional regulators (TTRs) directly regulate gene expression from the cell membrane. Prior work shows that two TTRs, TfoS and ChiS, cooperate to activate horizontal gene transfer by natural transformation in response to chitin in the facultative pathogen Vibrio cholerae . However, how these TTRs coordinate to activate this response has remained unclear. Here, we show that ChiS likely promotes TfoS-dependent activation of natural transformation by (1) relocalizing its target promoter to the membrane and (2) recruiting TfoS to the membrane proximal promoter through a direct interaction. Together, these results inform our understanding of both the V. cholerae chitin response and how TTR function can be impacted by their membrane localization.

Periplasmic chitooligosaccharide-binding protein requires a three-domain organization for substrate translocation

Periplasmic solute-binding proteins (SBPs) specific for chitooligosaccharides, (GlcNAc)n (n = 2, 3, 4, 5 and 6), are involved in the uptake of chitinous nutrients and the negative control of chitin signal transduction in Vibrios. Most translocation processes by SBPs across the inner membrane have been explained thus far by two-domain open/closed mechanism. Here we propose three-domain mechanism of the (GlcNAc)n translocation based on experiments using a recombinant VcCBP, SBP specific for (GlcNAc)n from Vibrio cholerae. X-ray crystal structures of unliganded or (GlcNAc)3-liganded VcCBP solved at 1.2-1.6 Å revealed three distinct domains, the Upper1, Upper2 and Lower domains for this protein. Molecular dynamics simulation indicated that the motions of the three domains are independent and that in the (GlcNAc)3-liganded state the Upper2/Lower interface fluctuated more intensively, compared to the Upper1/Lower interface. The Upper1/Lower interface bound two GlcNAc residues tightly, while the Upper2/Lower interface appeared to loosen and release the bound sugar molecule. The three-domain mechanism proposed here was fully supported by binding data obtained by thermal unfolding experiments and ITC, and may be applicable to other translocation systems involving SBPs belonging to the same cluster.

Recent advances in the metabolic pathways and microbial production of coenzyme Q

Coenzyme Q (CoQ) serves as an electron carrier in aerobic respiration and has become an interesting target for biotechnological production due to its antioxidative effect and benefits in supplementation to patients with various diseases. Here, we review discovery of the pathway with a particular focus on its superstructuration and regulation, and we summarize the metabolic engineering strategies for overproduction of CoQ by microorganisms. Studies in model microorganisms elucidated the details of CoQ biosynthesis and revealed the existence of multiprotein complexes composed of several enzymes that catalyze consecutive reactions in the CoQ pathways of Saccharomyces cerevisiae and Escherichia coli. Recent findings indicate that the identity and the total number of proteins involved in CoQ biosynthesis vary between species, which raises interesting questions about the evolution of the pathway and could provide opportunities for easier engineering of CoQ production. For the biotechnological production, so far only microorganisms have been used that naturally synthesize CoQ₁₀ or a related CoQ species. CoQ biosynthesis requires the aromatic precursor 4-hydroxybenzoic acid and the prenyl side chain that defines the CoQ species. Up to now, metabolic engineering strategies concentrated on the overproduction of the prenyl side chain as well as fine-tuning the expression of ubi genes from the ubiquinone modification pathway, resulting in high CoQ yields. With expanding knowledge about CoQ biosynthesis and exploration of new strategies for strain engineering, microbial CoQ production is expected to improve.