Recombinant Uncharacterized HTH-type transcriptional regulator Rv1816/MT1864 (Rv1816, MT1864)

What is Rv1816/MT1864 and what genetic features characterize this protein?

Rv1816 (also known as MT1864) is a transcriptional regulator belonging to the TetR family in Mycobacterium tuberculosis H37Rv. The gene encoding this protein is located at genomic position 2058256-2058960 on the positive strand, spanning 705 nucleotides and encoding a protein of 234 amino acids . Rv1816 contains a characteristic helix-turn-helix (HTH) motif at amino acid positions 38-59 (+4.30 SD), which is critical for its function as a DNA-binding transcriptional regulator . MEME analysis suggests similarity to other putative M. tuberculosis transcriptional regulators, specifically Rv0653c and Rv0681 .

The protein has been identified in the membrane protein fraction and whole cell lysates of M. tuberculosis H37Rv through mass spectrometry analysis, but it was not detected in culture filtrates . This subcellular localization is consistent with its predicted role in transcriptional regulation.

What is the predicted functional role of Rv1816 in Mycobacterium tuberculosis?

Rv1816 is primarily involved in transcriptional regulatory mechanisms in M. tuberculosis . As a member of the TetR family of transcriptional regulators, it likely functions as a repressor that undergoes conformational changes upon binding specific ligands, resulting in the release of the repressor from the operator and allowing transcription to proceed. The protein has been found to be required for bacterial growth on cholesterol as a carbon source , suggesting a potential role in lipid metabolism regulation.

Transcriptomics data shows that Rv1816 exhibits lower expression levels in wild-type M. tuberculosis H37Rv compared to phoP|Rv0757 mutant strains , indicating that its expression may be influenced by the PhoP regulatory system, which is a key two-component system involved in virulence regulation.

Is Rv1816 essential for M. tuberculosis survival and growth?

Multiple independent studies using Himar1 transposon mutagenesis in different strains (H37Rv and CDC1551) have consistently classified Rv1816 as a non-essential gene for in vitro growth of M. tuberculosis . Specifically:

• Non-essential for growth of H37Rv in MtbYM rich medium (Minato et al. 2019)

• Non-essential by analysis of saturated Himar1 transposon libraries (DeJesus et al. 2017)

• Non-essential in both H37Rv and CDC1551 strains (Sassetti et al., 2003; Lamichhane et al., 2003)

• Non-essential for in vitro growth of H37Rv (Griffin et al., 2011)

How does the co-regulation of Rv1816 with other genes provide insights into its function?

Rv1816 is predicted to be co-regulated in two primary modules: bicluster_0173 (residual 0.41) and bicluster_0443 (residual 0.45) . This co-regulation is potentially mediated by de-novo identified cis-regulatory motifs with different e-values in each module:

• Bicluster_0173: e-values of 11.00 and 12,000.00

• Bicluster_0443: e-values of 0.00 and 0.40

The genes co-regulated with Rv1816 in these modules are enriched for specific Gene Ontology terms including:

• Cellular biosynthetic processes

• Organic substance biosynthetic processes

• Cytoplasm-related functions

• RNA binding activities

This pattern of co-regulation suggests that Rv1816 may participate in metabolic and biosynthetic pathways, potentially regulating genes involved in cellular metabolism and RNA processing. The co-expression patterns provide valuable clues for identifying potential transcriptional targets of Rv1816 and understanding its regulatory network.

What is known about the binding properties and protein interactions of HTH-type transcriptional regulators like Rv1816?

While specific binding information for Rv1816 is limited in the provided search results, insights can be drawn from studies of similar HTH-type transcriptional regulators. In comparable systems, HTH-type transcriptional regulators have been shown to interact with other proteins with binding constants (Kᴅ) in the micromolar range. For instance, in Gloeobacter violaceus, the HTH-type transcriptional regulator GvTcR binds to its target promoters with dissociation constants of:

• 9.25 ± 1.3 μM for the ABC transporter ATP-binding protein coding promoter

• 19.72 ± 8.3 μM for the self-regulated GvTcR coding promoter

For protein-protein interactions, thermodynamic analysis has revealed that some HTH transcription regulators can bind to other proteins with dissociation constants around 8 ± 3.2 μM . The binding may induce conformational changes that affect spectroscopic properties, as observed by shifts in absorption spectra .

For Rv1816 specifically, its HTH motif (amino acids 38-59) is likely critical for DNA recognition and binding , allowing it to recognize specific operator sequences in the promoters of its target genes.

What are the optimal methods for expressing and purifying recombinant Rv1816 protein?

Based on general protocols for recombinant protein expression and the specific properties of TetR family regulators, the following methodological approach is recommended:

• Expression System Selection:

  • E. coli expression systems are commonly used for recombinant expression of mycobacterial proteins

  • For membrane-associated proteins like Rv1816, bacterial strains optimized for membrane protein expression (such as C41(DE3) or C43(DE3)) may yield better results

  • Expression vectors containing affinity tags (His6-tag, GST-tag) facilitate purification

• Expression Optimization:

  • Temperature: Lower temperatures (16-25°C) often yield better folding for transcriptional regulators

  • Induction: Optimized IPTG concentration (typically 0.1-0.5 mM)

  • Duration: Extended expression periods (16-20 hours) at lower temperatures

• Purification Strategy:

  • Initial affinity chromatography (Ni-NTA for His-tagged proteins)

  • Size exclusion chromatography to ensure monomer content >99%

  • Ion exchange chromatography may be required for higher purity

• Quality Control:

  • High-performance size exclusion chromatography to confirm monomer content

  • SDS-PAGE and Western blotting for purity assessment

  • Mass spectrometry for identity confirmation (as applied in previous studies of Rv1816)

For specific biochemical studies, such as DNA-binding assays, the protein should be maintained in buffers optimized to preserve the native conformation of the HTH motif.

How can researchers effectively study the DNA-binding properties and specificity of Rv1816?

To characterize the DNA-binding properties of Rv1816, researchers can employ multiple complementary approaches:

• Electrophoretic Mobility Shift Assays (EMSA):

  • Use purified recombinant Rv1816 protein

  • Test binding to predicted promoter regions based on co-regulation data

  • Determine binding affinity and specificity through competition assays

  • Incorporate fluorescent or radioactive labeling for enhanced sensitivity

• DNase I Footprinting:

  • Identify specific DNA sequences protected by Rv1816 binding

  • Map the precise operator sequences within target promoters

  • Compare binding sites with in silico predicted motifs from the co-regulation data

• Chromatin Immunoprecipitation (ChIP-seq):

  • Generate antibodies against Rv1816 or use epitope-tagged versions

  • Identify genome-wide binding sites in vivo

  • Correlate with expression data from the existing datasets

• Surface Plasmon Resonance (SPR) or Isothermal Titration Calorimetry (ITC):

  • Determine binding kinetics and thermodynamic parameters

  • Quantify dissociation constants (Kᴅ) for different DNA targets

  • Similar techniques have been applied to related HTH regulators with Kᴅ values in the micromolar range

• Reporter Gene Assays:

  • Construct reporter systems with predicted binding sites

  • Evaluate the functional consequences of Rv1816 binding

  • Test effects of mutations in the HTH motif (amino acids 38-59)

These methods would help elucidate the specific DNA sequences recognized by Rv1816 and its regulatory effects on target genes.

What methods are appropriate for assessing the thermostability of recombinant Rv1816?

Several complementary techniques can be employed to evaluate the thermostability of Rv1816:

• Right Angle Light Scattering (RALS):

  • Uses standard fluorescence spectrometer equipment

  • Excitation at λᴇₓ = 320 nm and emission recorded at λᴇₘ = 320 nm

  • Temperature ramping from 20°C to 90°C using controlled protocols

  • Monitors protein aggregation during thermal denaturation

• Intrinsic Tryptophan Fluorescence (ITF):

  • Can be performed concurrently with RALS using the same equipment

  • Excitation at λᴇₓ = 295 nm with emission recorded between λᴇₘ = 300-400 nm

  • Measures environmental changes around tryptophan residues

  • Results typically plotted as intensity ratios (e.g., 350/330) against temperature

• Differential Scanning Calorimetry (DSC):

  • Direct measurement of heat capacity changes during protein unfolding

  • Provides thermodynamic parameters (ΔH, ΔS, ΔG)

  • Allows determination of precise melting temperatures (Tm)

• Circular Dichroism (CD) Spectroscopy:

  • Monitors changes in secondary structure during thermal denaturation

  • Particularly useful for analyzing the HTH motif stability

  • Temperature-dependent spectra can reveal unfolding transitions

• Thermal Shift Assays (TSA):

  • Uses fluorescent dyes (e.g., SYPRO Orange) that bind to hydrophobic regions

  • High-throughput screening of buffer conditions affecting stability

  • Suitable for optimization of storage and experimental conditions

These methods provide complementary data about different aspects of protein stability and can help identify conditions that maintain the functional integrity of Rv1816.

How do mutations in the HTH motif affect the structure and function of Rv1816?

While specific mutation data for Rv1816 is not directly available in the search results, insights can be drawn from studies of related HTH-type transcriptional regulators. For HTH-type regulators in general:

• Critical Residues in the HTH Motif:

  • Positively charged residues (arginine, lysine) in the recognition helix are often crucial for DNA binding

  • Mutations of these residues can significantly impact DNA binding affinity

  • For example, in related systems, R69A/K141A double mutations increased dissociation constants by approximately 10³-fold

• Spectroscopic Changes:

  • Mutations affecting protein-protein interactions may eliminate spectral shifts observed in wild-type proteins

  • For example, R69A/K141A mutations in a related system prevented the spectral shift from 538 nm to 533 nm typically seen upon protein binding

• Gel Electrophoresis Analysis:

  • Protein-protein interactions may cause band shifts in gel electrophoresis

  • Mutations disrupting these interactions can eliminate such shifts

  • This approach can be used to validate the effects of specific mutations

For Rv1816 specifically, targeted mutations within the HTH motif (amino acids 38-59) would likely affect:

• DNA-binding specificity and affinity

• Protein stability and folding

• Interactions with potential protein partners

• Regulatory function in transcriptional control

Systematic mutagenesis studies would be valuable for mapping the structure-function relationships in Rv1816.

How can transcriptomic and proteomic approaches be used to identify the regulon of Rv1816?

To comprehensively identify genes regulated by Rv1816, researchers can employ multiple omics approaches:

• Comparative Transcriptomics:

  • RNA-seq or microarray analysis comparing wild-type and Rv1816 knockout/overexpression strains

  • Analysis under different growth conditions, particularly cholesterol utilization

  • Integration with existing transcriptomics data showing differential expression in phoP mutants

  • Time-course experiments to capture direct vs. indirect regulatory effects

• ChIP-seq Analysis:

  • Genome-wide mapping of Rv1816 binding sites in vivo

  • Integration with transcriptomics data to correlate binding with expression changes

  • Analysis of binding site motifs to determine consensus sequences

  • Comparison with predicted co-regulated gene clusters (bicluster_0173 and bicluster_0443)

• Proteomics:

  • Quantitative proteomics comparing protein levels in wild-type vs. Rv1816 mutant strains

  • Building on existing proteomic data that identified Rv1816 in membrane fractions

  • Analysis of post-translational modifications affecting Rv1816 activity

  • Protein-protein interaction studies using pull-down or co-immunoprecipitation approaches

• Integration with Existing Datasets:

  • Correlation with genes enriched in GO terms identified in co-regulation studies:

    • Cellular biosynthetic processes

    • Organic substance biosynthetic processes

    • Cytoplasm-related functions

    • RNA binding activities

  • Analysis of expression patterns in existing ChIP-seq and TFOE (transcription factor overexpression) datasets

• Metabolomics:

  • Given Rv1816's role in cholesterol utilization , metabolite profiling could reveal pathways under its control

  • Focused analysis on lipid metabolites and intermediates of cholesterol catabolism

This multi-omics approach would provide a comprehensive view of the Rv1816 regulon and its role in M. tuberculosis physiology.

What is the significance of Rv1816's role in cholesterol metabolism for M. tuberculosis pathogenesis?

The requirement of Rv1816 for growth on cholesterol has important implications for M. tuberculosis pathogenesis:

• Cholesterol as a Critical Carbon Source During Infection:

  • M. tuberculosis can utilize host cholesterol as a carbon and energy source during infection

  • Cholesterol metabolism is particularly important during the chronic phase of infection

  • Genes involved in cholesterol utilization are often upregulated during macrophage infection

• Potential Role in Host-Pathogen Interactions:

  • As a transcriptional regulator required for cholesterol utilization, Rv1816 may control genes involved in:

    • Cholesterol uptake and transport

    • Breakdown of sterol ring structures

    • Metabolism of cholesterol side chains

    • Integration of cholesterol-derived metabolites into central metabolism

• Connection to Virulence and Persistence:

  • Cholesterol metabolism genes are often required for full virulence in animal models

  • The non-essentiality of Rv1816 in standard in vitro conditions but its requirement for cholesterol utilization suggests a specialized role during host adaptation

  • This pattern is consistent with genes involved in pathogenesis rather than basic cellular functions

• Therapeutic Implications:

  • Inhibition of Rv1816 function could potentially attenuate bacterial persistence in cholesterol-rich environments

  • Understanding its regulon could identify novel drug targets in cholesterol metabolism pathways

  • The availability of Rv1816 mutants facilitates experimental validation of these concepts

• Connection to Other Regulatory Systems:

  • The differential expression of Rv1816 in phoP mutants suggests crosstalk between different regulatory systems

  • This may indicate integration of multiple environmental signals (including availability of different carbon sources) in regulating bacterial adaptation

Further research into how Rv1816 regulates cholesterol metabolism could provide insights into M. tuberculosis adaptation strategies during infection.

How can protein-protein interactions involving Rv1816 be identified and characterized in mycobacterial systems?

To investigate potential protein-protein interactions involving Rv1816, researchers can employ several complementary approaches:

• Bacterial Two-Hybrid (B2H) Systems:

  • Adaptation of yeast two-hybrid for bacterial proteins

  • Screening of M. tuberculosis genomic libraries for potential interactors

  • Validation of specific interactions with candidate partners

• Pull-Down Assays with Tagged Rv1816:

  • Expression of affinity-tagged Rv1816 in mycobacterial cells

  • Affinity purification followed by mass spectrometry identification

  • Building on existing proteomics approaches that identified Rv1816 in membrane fractions

• Co-Immunoprecipitation (Co-IP):

  • Development of specific antibodies against Rv1816 or use of epitope tags

  • Precipitation of protein complexes from mycobacterial lysates

  • Western blot or mass spectrometry analysis of co-precipitated proteins

• Isothermal Titration Calorimetry (ITC):

  • Quantitative measurement of binding affinities between purified proteins

  • Determination of thermodynamic parameters of interactions

  • Similar approaches with related HTH regulators have revealed Kᴅ values of approximately 8 μM

• Spectroscopic Analysis of Protein Complexes:

  • Monitoring spectral shifts upon protein binding

  • For example, blue shifts from 538 nm to 533 nm have been observed in related systems

  • Detection of gel shift patterns in electrophoresis as observed with related proteins

• Biolayer Interferometry or Surface Plasmon Resonance:

  • Real-time measurement of binding kinetics

  • Determination of association and dissociation rates

  • Screening of multiple potential interacting partners

• Crosslinking Mass Spectrometry:

  • In vivo crosslinking to capture transient interactions

  • MS/MS analysis to identify crosslinked peptides

  • Mapping of interaction interfaces

These methods would help elucidate the protein interaction network of Rv1816 and provide insights into its regulatory mechanisms beyond direct DNA binding.

What are the challenges and solutions in studying the effect of environmental signals on Rv1816 activity?

Investigating how environmental signals modulate Rv1816 activity presents several challenges with corresponding methodological solutions:

• Challenge: Identifying Relevant Environmental Signals

  Solutions:

  • Systematic testing of Rv1816 expression and activity under various stress conditions relevant to infection (hypoxia, nutrient limitation, acidic pH, etc.)

  • Correlation with in vivo expression data from infection models

  • Focus on cholesterol-related conditions given Rv1816's role in cholesterol utilization

  • Integration with PhoP regulon data, since Rv1816 shows differential expression in phoP mutants

• Challenge: Monitoring Rv1816 Activity in Real-Time

  Solutions:

  • Development of reporter systems with Rv1816-controlled promoters

  • Fluorescent or luminescent readouts to track activity changes

  • Similar to approaches used in studying light-regulated ABC transporters in other systems

  • Time-course transcriptomics or proteomics to capture dynamics of the Rv1816 regulon

• Challenge: Distinguishing Direct vs. Indirect Effects

  Solutions:

  • Combination of ChIP-seq with RNA-seq at multiple time points

  • Use of rapidly inducible expression systems

  • Protein synthesis inhibition experiments to identify direct regulatory targets

  • Analysis of the kinetics of gene expression changes

• Challenge: Identifying Potential Ligands Affecting Rv1816 Function

  Solutions:

  • Thermal shift assays screening metabolite libraries for stabilizing effects

  • Isothermal titration calorimetry to quantify binding affinities

  • Structural studies (X-ray crystallography, cryo-EM) with and without ligands

  • Focus on cholesterol intermediates and related compounds

• Challenge: Physiological Relevance of in vitro Findings

  Solutions:

  • Validation in mycobacterial cell culture systems

  • Generation of point mutations affecting ligand binding but not DNA binding

  • Macrophage infection models to test relevance during host-pathogen interaction

  • Animal models comparing wild-type and Rv1816 mutant strains

• Challenge: Technical Limitations in Mycobacterial Systems

  Solutions:

  • Development of inducible knockdown systems for essential pathways

  • Use of surrogate mycobacterial hosts (M. smegmatis) for initial studies

  • Adaptation of methods from studies of related transcriptional regulators

  • Application of light-inducible systems similar to those described for other regulators

Addressing these challenges would provide a comprehensive understanding of how environmental signals regulate Rv1816 activity and its downstream effects on M. tuberculosis physiology.