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Researchers demonstrated that a single helix-turn-helix (HTH) motif from the MarA transcription factor, rather than the traditionally required two-motif structure, can bind DNA and activate gene transcription when a specific seven-amino acid sequence (IRSRKMT) is present. The study used biochemical assays, molecular dynamics simulations, and structural modeling to show that this seven-residue extension enables proper dimerization of the minimal HTH domain on DNA, correctly positioning it to interact with RNA polymerase. Without these seven amino acids, the truncated protein still binds DNA but forms non-productive complexes that actually suppress gene expression below baseline levels.
Why it matters
This work reveals the minimal functional requirements for bacterial transcription factors involved in antibiotic resistance, which could inform the development of anti-virulence therapeutics that disrupt these proteins. The findings also provide a simplified, tunable protein scaffold for synthetic biology applications requiring designed transcriptional control systems.
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⚠️ Preprint – Noch nicht peer-reviewed
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Prokaryotic transcription factors (TFs) lie at the core of antimicrobial resistance, controlling genes that let bacteria survive antibiotic exposure. The AraC/XylS family of TFs is defined by a ~99-residue DNA-binding domain composed of two helix-turn-helix (HTH) motifs. While this two-motif architecture is considered the minimal functional unit, the striking sequence and structural similarity between both HTH motifs raises the question of whether it evolved from a single ancestral HTH domain. Here, we designed two C-terminal truncations of MarA, comprising a single HTH motif, differing by seven-residues (IRSRKMT). Electrophoretic-mobility shift assays reveal that both constructs specifically bind the marbox sequence as reconstituted dimers, while size-exclusion chromatography shows they exist in a monomer-dimer equilibrium in solution. Despite retaining DNA-binding capacity, the truncations diverge functionally: while MarA64 (including IRSRKMT) activates transcription and confers regular erythromycin tolerance, MarA57 (lacking IRSRKMT) yields a transcriptionally inactive complex that suppresses reporter expression below baseline, suggesting competitive promoter occupancy. Molecular dynamics simulations and AlphaFold models suggest that the IRSRKMT extension forms an -helical element stabilizing a transcriptionally productive dimer interface. Conversely, its loss disrupts quaternary assembly, alters DNA bending, and misaligns RNA polymerase-contacting residues. Furthermore, free dimers explore non-productive conformations, suggesting that functional dimerization occurs upon DNA engagement. These findings establish that a single, correctly dimerized HTH domain is sufficient for both DNA binding and transcriptional activation, providing a structural rationale for short AraC/XylS-like proteins and offering a tuneable scaffold for synthetic biology and novel anti-virulence strategies.