In Vivo Evidence for TonB Dimerization
Bibliographic record
Abstract
TonB, in complex with ExbB and ExbD, is required for the energy-dependent transport of ferric siderophores across the outer membrane of Escherichia coli, the killing of cells by group B colicins, and infection by phages T1 and phi80. To gain insights into the protein complex, TonB dimerization was studied by constructing hybrid proteins from complete TonB (containing amino acids 1 to 239) [TonB(1-239)] and the cytoplasmic fragment of ToxR which, when dimerized, activates the transcription of the cholera toxin gene ctx. ToxR(1-182)-TonB(1-239) activated the transcription of lacZ under the control of the ctx promoter (P(ctx)::lacZ). Replacement of the TonB transmembrane region by the ToxR transmembrane region resulted in the hybrid proteins ToxR(1-210)-TonB(33-239) and ToxR(1-210)-TonB(164-239), of which only the latter activated P(ctx)::lacZ transcription. Dimer formation was reduced but not abolished in a mutant lacking ExbB and ExbD, suggesting that these complex components may influence dimerization but are not strictly required and that the N-terminal cytoplasmic membrane anchor and the C-terminal region are important for dimer formation. The periplasmic TonB fragment, TonB(33-239), inhibits ferrichrome and ferric citrate transport and induction of the ferric citrate transport system. This competition provided a means to positively screen for TonB(33-239) mutants which displayed no inhibition. Single point mutations of inactive fragments selected in this manner were introduced into complete TonB, and the phenotypes of the TonB mutant strains were determined. The mutations located in the C-terminal half of TonB, three of which (Y163C, V188E, and R204C) were obtained separately by site-directed mutagenesis, as was the isolated F230V mutation, were studied in more detail. They displayed different activity levels for various TonB-dependent functions, suggesting function-related specificities which reflect differences in the interactions of TonB with various transporters and receptors.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".