Structural and Functional Investigation of the Periplasmic Arsenate-Binding Protein ArrX from Chrysiogenes arsenatis

Marine picocyanobacterial PhnD1 shows specificity for various phosphorus sources but likely represents a constitutive inorganic phosphate transporter

Despite being fundamental to multiple biological processes, phosphorus (P) availability in marine environments is often growth-limiting, with generally low surface concentrations. Picocyanobacteria strains encode a putative ABC-type phosphite/phosphate/phosphonate transporter, phnDCE, thought to provide access to an alternative phosphorus pool. This, however, is paradoxical given most picocyanobacterial strains lack known phosphite degradation or carbon-phosphate lyase pathway to utilise alternate phosphorus pools. To understand the function of the PhnDCE transport system and its ecological consequences, we characterised the PhnD1 binding proteins from four distinct marine Synechococcus isolates (CC9311, CC9605, MITS9220, and WH8102). We show the Synechococcus PhnD1 proteins selectively bind phosphorus compounds with a stronger affinity for phosphite than for phosphate or methyl phosphonate. However, based on our comprehensive ligand screening and growth experiments showing Synechococcus strains WH8102 and MITS9220 cannot utilise phosphite or methylphosphonate as a sole phosphorus source, we hypothesise that the picocyanobacterial PhnDCE transporter is a constitutively expressed, medium-affinity phosphate transporter, and the measured affinity of PhnD1 to phosphite or methyl phosphonate is fortuitous. Our MITS9220_PhnD1 structure explains the comparatively lower affinity of picocyanobacterial PhnD1 for phosphate, resulting from a more limited H-bond network. We propose two possible physiological roles for PhnD1. First, it could function in phospholipid recycling, working together with the predicted phospholipase, TesA, and alkaline phosphatase. Second, by having multiple transporters for P (PhnDCE and Pst), picocyanobacteria could balance the need for rapid transport during transient episodes of higher P availability in the environment, with the need for efficient P utilisation in typical phosphate-deplete conditions.

Ultrasensitive Reagent for Ratiometric Detection and Detoxification of III in Water and Mitochondria

Toxicity induced by inorganic arsenic as AsO₃^3- (iAs^III) is of global concern. Reliable detection of the maximum allowed contaminant level for arsenic in drinking water and in the cellular system remains a challenge for the water quality management and assessment of toxicity in the cellular milieu, respectively. A new Ir(III)-based phosphorescent molecule (AS-1; λ_Ext = 415 nm and λ_Ems = 600 nm, Φ = 0.3) is synthesized for the selective detection of iAs^III in an aqueous solution with a ratiometric luminescence response even in the presence of iAs^V and all other common inorganic cations and anions. The relatively higher affinity of the thioimidazole ligand (HPBT) toward iAs^III led to the formation of a fluorescent molecule iAs^V-HPBT (λ_Ext = 415 nm and λ_Ems = 466 nm, Φ = 0.28) for the reaction of iAs^III and AS-1. An improved limit of quantitation (LOQ) down to 0.2 ppb is achieved when AS-1 is used in the CTAB micellar system. Presumably, the cationic surfactants favor the localization of AS-1@CTAB^Micelle in mitochondria of MCF7 cells, and this is confirmed from the images of the confocal laser fluorescence scanning microscopic studies. Importantly, cell viability assay studies confirm that AS-1@CTAB^Micelle induces dose-dependent detoxification of iAs^III in live cells. Further, luminescence responses at 466 nm could be utilized for developing a hand-held device for the in-field application. Such a reagent that allows for ratiometric detection of iAs^III with LOQ of 2.6 nM (0.5 ppb) in water, as well as helps in visualizing its distribution in mitochondria with a detoxifying effect, is rather unique in contemporary literature.

Sensory perception in bacterial cyclic diguanylate signal transduction

Cyclic diguanylate (c-di-GMP) signal transduction systems provide bacteria with the ability to sense changing cell status or environmental conditions and then execute suitable physiological and social behaviors in response. In this review, we provide a comprehensive census of the stimuli and receptors that are linked to the modulation of intracellular c-di-GMP. Emerging evidence indicates that c-di-GMP networks sense light, surfaces, energy, redox potential, respiratory electron acceptors, temperature, and structurally diverse biotic and abiotic chemicals. Bioinformatic analysis of sensory domains in diguanylate cyclases and c-di-GMP-specific phosphodiesterases as well as the receptor complexes associated with them reveals that these functions are linked to a diverse repertoire of protein domain families. We describe the principles of stimulus perception learned from studying these modular sensory devices, illustrate how they are assembled in varied combinations with output domains, and summarize a system for classifying these sensor proteins based on their complexity. Biological information processing via c-di-GMP signal transduction not only is fundamental to bacterial survival in dynamic environments but also is being used to engineer gene expression circuitry and synthetic proteins with à la carte biochemical functionalities.