OFF-switching property of quorum sensor LuxR via As(III)-induced insoluble form

Enhancement of Sensitivity in Aggregation-Based Whole-Cell Arsenite Sensor Utilizing Arsenic Metabolism Regulation

Arsenite [As(III)] is a toxic substance widely present on Earth, and the development of low-cost and simple microbial-based As(III) sensors has been attracting attention. Recently, we discovered that the protein LuxR, which contains multiple cysteine residues with high affinity for As(III), forms an insoluble structure upon binding to As(III) and exhibits OFF-switching properties as a quorum sensing transcriptional activator. Based on this property, the LuxR sensor operates on a new principle distinct from conventional whole-cell As(III) sensors; however, its sensitivity remains a challenge. In this study, we aimed to improve the sensitivity of the whole-cell OFF-type As(III) sensor by increasing the frequency of intracellular interactions between the sensor protein and As(III). We utilized the super-repressor properties of ArsR, a transcriptional repressor of the As(III)-metabolizing ars operon, achieved by replacing C34 in its As(III)-binding domain with Y. By linking ArsRC34Y with the OFF-type As(III) sensor protein LuxR, we constructed a single plasmid to create a portable ArsRC34Y-LuxR sensor protein. By suppressing the expression of ArsB, an As(III) efflux transporter encoded in the ars operon, using ArsRC34Y, we successfully enhanced the sensitivity of the OFF-type As(III) response.

Imparting As(III) Responsiveness to the Choline Response Transcriptional Regulator BetI

The development of a low-cost and user-friendly sensor using microorganisms to monitor the presence of As(III) on earth has garnered significant attention. In conventional research on microbial As(III) sensors, the focus has been on transcription factor ArsR, which plays a role in As(III) metabolism. However, we recently discovered that LuxR, a quorum-sensing control factor in Vibrio fischeri that contains multiple cysteine residues, acted as an As(III) sensor despite having no role in As(III) metabolism. This finding suggested that any protein could be an As(III) sensor if cysteine residues were incorporated. In this study, we aimed to confer As(III) responsiveness to BetI, a transcriptional repressor of the TetR family involved in osmotic regulation of the choline response, unrelated to As(III) metabolism. Based on the BetI structure constructed using molecular dynamics calculations, we generated a series of mutants in which each of the three amino acids not critical for function was substituted with cysteine. Subsequent examination of their response to As(III) revealed that the cysteine-substituted mutant, incorporating all three substitutions, demonstrated As(III) responsiveness. This was evidenced by the fluorescence intensity of the downstream reporter superfolder green fluorescent protein expression regulated by the operator region. Intriguingly, the BetI cysteine mutant maintained its binding responsiveness to the natural ligand choline. We successfully engineered an OR logic gate capable of responding to two orthogonal ligands using a single protein.

Improvement of a highly sensitive and specific whole-cell biosensor by adding a positive feedback amplifier

In this study, we designed a Cd²⁺ whole-cell biosensor with both positive and negative feedback cascade amplifiers in Pseudomonas putida KT2440 (LTCM) based on our previous design with only a negative feedback amplifier (TCM). The results showed that the newly developed biosensor LTCM was greatly improved compared to TCM. Firstly, the linear response range of LTCM was expanded while the maximum linear response range was raised from 0.05 to 0.1 μM. Meanwhile, adding a positive feedback amplifier further increased the fluorescence output signal of LTCM 1.11-2.64 times under the same culture conditions. Moreover, the response time of LTCM for detection of practical samples was reduced from 6 to 4 h. At the same time, LTCM still retained very high sensitivity and specificity, while its lowest detection limit was 0.1 nM Cd²⁺ and the specificity was 23.29 (compared to 0.1 nM and 17.55 in TCM, respectively). In summary, the positive and negative feedback cascade amplifiers effectively improved the performance of the biosensor LTCM, resulting in a greater linear response range, higher output signal intensity, and shorter response time than TCM while retaining comparable sensitivity and specificity, indicating better potential for practical applications.

Research Progress in Fluorescent Probes for Arsenic Species

Arsenic is a toxic non-metallic element that is widely found in nature. In addition, arsenic and arsenic compounds are included in the list of Group I carcinogens and toxic water pollutants. Therefore, rapid and efficient methods for detecting arsenic are necessary. In the past decade, a variety of small molecule fluorescent probes have been developed, which has been widely recognized for their rapidness, efficiency, convenience and sensitivity. With the development of new nanomaterials (AuNPs, CDs and QDs), organic molecules and biomolecules, the conventional detection of arsenic species based on fluorescence spectroscopy is gradually transforming from the laboratory to the portable kit. Therefore, in view of the current research status, this review introduces the research progress of both traditional and newly developed fluorescence spectrometry based on novel materials for arsenic detection, and discusses the potential of this technology in the rapid screening and field testing of water samples contaminated with arsenic. The review also discusses the problems that still exist in this field, as well as the expectations.