Structural Analysis of the Phenol-Responsive Sensory Domain of the Transcription Activator PoxR

The eukaryotic-like characteristics of small GTPase, roadblock and TRAPPC3 proteins from Asgard archaea

Membrane-enclosed organelles are defining features of eukaryotes in distinguishing these organisms from prokaryotes. Specification of distinct membranes is critical to assemble and maintain discrete compartments. Small GTPases and their regulators are the signaling molecules that drive membrane-modifying machineries to the desired location. These signaling molecules include Rab and Rag GTPases, roadblock and longin domain proteins, and TRAPPC3-like proteins. Here, we take a structural approach to assess the relatedness of these eukaryotic-like proteins in Asgard archaea, the closest known prokaryotic relatives to eukaryotes. We find that the Asgard archaea GTPase core domains closely resemble eukaryotic Rabs and Rags. Asgard archaea roadblock, longin and TRAPPC3 domain-containing proteins form dimers similar to those found in the eukaryotic TRAPP and Ragulator complexes. We conclude that the emergence of these protein architectures predated eukaryogenesis, however further adaptations occurred in proto-eukaryotes to allow these proteins to regulate distinct internal membranes.

Sensitive and selective phenol sensing in denitrifying Aromatoleum aromaticum EbN1T

IMPORTANCE

Aromatic compounds are globally abundant organic molecules with a multitude of natural and anthropogenic sources, underpinning the relevance of their biodegradation. A. aromaticum EbN1T is a well-studied environmental betaproteobacterium specialized on the anaerobic degradation of aromatic compounds. The here studied responsiveness toward phenol in conjunction with the apparent high ligand selectivity (non-promiscuity) of its PheR sensor and those of the related p-cresol (PcrS) and p-ethylphenol (EtpR) sensors are in accord with the substrate-specificity and biochemical distinctiveness of the associated degradation pathways. Furthermore, the present findings advance our general understanding of the substrate-specific regulation of the strain's remarkable degradation network and of the concentration thresholds below which phenolic compounds become essentially undetectable and as a consequence should escape substantial biodegradation. Furthermore, the findings may inspire biomimetic sensor designs for detecting and quantifying phenolic contaminants in wastewater or environments.

Identifying Selectivity Filters in Protein Biosensor for Ligand Screening

Specialized sensing mechanisms in bacteria enable the identification of cognate ligands with remarkable selectivity in highly xenobiotic-polluted environments where these ligands are utilized as energy sources. Here, via integrating all-atom computer simulation, biochemical assay, and isothermal titration calorimetry measurements, we determine the molecular basis of MopR, a phenol biosensor's complex selection process of ligand entry. Our results reveal a set of strategically placed selectivity filters along the ligand entry pathway of MopR. These filters act as checkpoints, screening diverse aromatic ligands at the protein surface based on their chemical features and sizes. Ligands meeting specific criteria are allowed to enter the sensing site in an orientation-dependent manner. Sequence and structural analyses demonstrate the conservation of this ligand entry mechanism across the sensor class, with individual amino acids along the selectivity filter path playing a critical role in ligand selection. Together, this investigation highlights the importance of interactions with the ligand entry pathway, in addition to interactions within the binding pocket, in achieving ligand selectivity in biological sensing. The findings enhance our understanding of ligand selectivity in bacterial phenol biosensors and provide insights for rational expansion of the biosensor repertoire, particularly for the biotechnologically relevant class of aromatic pollutants.

Cooperativity in ATP Hydrolysis by MopR Is Modulated by Its Signal Reception Domain and by Its Protein and Phenol Concentrations

The NtrC family of AAA+ proteins are bacterial transcriptional regulators that control σ54-dependent RNA polymerase transcription under certain stressful conditions. MopR, which is a member of this family, is responsive to phenol and stimulates its degradation. Biochemical studies to understand the role of ATP and phenol in oligomerization and allosteric regulation, which are described here, show that MopR undergoes concentration-dependent oligomerization in which dimers assemble into functional hexamers. The oligomerization occurs in a nucleation-dependent manner with a tetrameric intermediate. Additionally, phenol binding is shown to be responsible for shifting MopR's equilibrium from a repressed state (high affinity toward ATP) to a functionally active, derepressed state with low-affinity for ATP. Based on these findings, we propose a model for allosteric regulation of MopR. IMPORTANCE The NtrC family of bacterial transcriptional regulators are enzymes with a modular architecture that harbor a signal sensing domain followed by a AAA+ domain. MopR, a NtrC family member, responds to phenol and activates phenol adaptation pathways that are transcribed by σ54-dependent RNA polymerases. Our results show that for efficient ATP hydrolysis, MopR assembles as functional hexamers and that this activity of MopR is regulated by its effector (phenol), ATP, and protein concentration. Our findings, and the kinetic methods we employ, should be useful in dissecting the allosteric mechanisms of other AAA+ proteins, in general, and NtrC family members in particular.

Tetrameric architecture of an active phenol-bound form of the AAA+ transcriptional regulator DmpR

The Pseudomonas putida phenol-responsive regulator DmpR is a bacterial enhancer binding protein (bEBP) from the AAA⁺ ATPase family. Even though it was discovered more than two decades ago and has been widely used for aromatic hydrocarbon sensing, the activation mechanism of DmpR has remained elusive. Here, we show that phenol-bound DmpR forms a tetramer composed of two head-to-head dimers in a head-to-tail arrangement. The DmpR-phenol complex exhibits altered conformations within the C-termini of the sensory domains and shows an asymmetric orientation and angle in its coiled-coil linkers. The structural changes within the phenol binding sites and the downstream ATPase domains suggest that the effector binding signal is propagated through the coiled-coil helixes. The tetrameric DmpR-phenol complex interacts with the σ⁵⁴ subunit of RNA polymerase in presence of an ATP analogue, indicating that DmpR-like bEBPs tetramers utilize a mechanistic mode distinct from that of hexameric AAA⁺ ATPases to activate σ⁵⁴-dependent transcription.

The Y233 gatekeeper of DmpR modulates effector-responsive transcriptional control of σ54 -RNA polymerase

DmpR is the obligate transcriptional activator of genes involved in (methyl)phenol catabolism by Pseudomonas putida. DmpR belongs to the AAA⁺ class of mechano-transcriptional regulators that employ ATP-hydrolysis to engage and remodel σ⁵⁴ -RNA polymerase to allow transcriptional initiation. Previous work has established that binding of phenolic effectors by DmpR is a prerequisite to relieve interdomain repression and allow ATP-binding to trigger transition to its active multimeric conformation, and further that a structured interdomain linker between the effector- and ATP-binding domains is involved in coupling these processes. Here, we present evidence from ATPase and in vivo and in vitro transcription assays that a tyrosine residue of the interdomain linker (Y233) serves as a gatekeeper to constrain ATP-hydrolysis and aromatic effector-responsive transcriptional activation by DmpR. An alanine substitution of Y233A results in both increased ATPase activity and enhanced sensitivity to aromatic effectors. We propose a model in which effector-binding relocates Y233 to synchronize signal-reception with multimerisation to provide physiologically appropriate sensitivity of the transcriptional response. Given that Y233 counterparts are present in many ligand-responsive mechano-transcriptional regulators, the model is likely to be pertinent for numerous members of this family and has implications for development of enhanced sensitivity of biosensor used to detect pollutants.

Phyletic Distribution and Lineage-Specific Domain Architectures of Archaeal Two-Component Signal Transduction Systems

The two-component signal transduction (TCS) machinery is a key mechanism of sensing environmental changes in the prokaryotic world. TCS systems have been characterized thoroughly in bacteria but to a much lesser extent in archaea. Here, we provide an updated census of more than 2,000 histidine kinases and response regulators encoded in 218 complete archaeal genomes, as well as unfinished genomes available from metagenomic data. We describe the domain architectures of the archaeal TCS components, including several novel output domains, and discuss the evolution of the archaeal TCS machinery. The distribution of TCS systems in archaea is strongly biased, with high levels of abundance in haloarchaea and thaumarchaea but none detected in the sequenced genomes from the phyla Crenarchaeota, Nanoarchaeota, and Korarchaeota The archaeal sensor histidine kinases are generally similar to their well-studied bacterial counterparts but are often located in the cytoplasm and carry multiple PAS and/or GAF domains. In contrast, archaeal response regulators differ dramatically from the bacterial ones. Most archaeal genomes do not encode any of the major classes of bacterial response regulators, such as the DNA-binding transcriptional regulators of the OmpR/PhoB, NarL/FixJ, NtrC, AgrA/LytR, and ActR/PrrA families and the response regulators with GGDEF and/or EAL output domains. Instead, archaea encode multiple copies of response regulators containing either the stand-alone receiver (REC) domain or combinations of REC with PAS and/or GAF domains. Therefore, the prevailing mechanism of archaeal TCS signaling appears to be via a variety of protein-protein interactions, rather than direct transcriptional regulation.IMPORTANCE Although the Archaea represent a separate domain of life, their signaling systems have been assumed to be closely similar to the bacterial ones. A study of the domain architectures of the archaeal two-component signal transduction (TCS) machinery revealed an overall similarity of archaeal and bacterial sensory modules but substantial differences in the signal output modules. The prevailing mechanism of archaeal TCS signaling appears to involve various protein-protein interactions rather than direct transcription regulation. The complete list of histidine kinases and response regulators encoded in the analyzed archaeal genomes is available online at http://www.ncbi.nlm.nih.gov/Complete_Genomes/TCSarchaea.html.

Structure Guided Design of Protein Biosensors for Phenolic Pollutants

Phenolic aromatic compounds are a major source of environmental pollution. Currently there are no in situ methods for specifically and selectively detecting these pollutants. Here, we exploit the nature's biosensory machinery by employing Acinetobacter calcoaceticus NCIB8250 protein, MopR, as a model system to develop biosensors for selective detection of a spectrum of these pollutants. The X-ray structure of the sensor domain of MopR was used as a scaffold for logic-based tunable biosensor design. By employing a combination of in silico structure guided approaches, mutagenesis and isothermal calorimetric studies, we were able to generate biosensor templates, that can selectively and specifically sense harmful compounds like chlorophenols, cresols, catechol, and xylenols. Furthermore, the ability of native protein to selectively sense phenol as the primary ligand was also enhanced. Overall, this methodology can be extended as a suitable framework for development of a series of exclusive biosensors for accurate and selective detection of aromatic pollutants from real time environmental samples.