Biosynthesis of the Methylthioxylose Capping Motif of Lipoarabinomannan in <i>Mycobacterium tuberculosis</i>
Bibliographic record
Abstract
Lipoarabinomannan (LAM) is a lipoglycan found in abundant quantities in the cell envelope of all mycobacteria. The nonreducing arabinan termini of LAM display species-specific structural microheterogeneity that impacts the biological activity of the entire molecule. Mycobacterium tuberculosis, for instance, produces mannoside caps made of one to three α-(1 → 2)-Manp-linked residues that may be further substituted with an α-(1 → 4)-linked methylthio-d-xylose (MTX) residue. While the biological functions and catalytic steps leading to the formation of the mannoside caps of M. tuberculosis LAM have been well established, the biosynthetic origin and biological relevance of the MTX motif remain elusive. We here report on the discovery of a five-gene cluster dedicated to the biosynthesis of the MTX capping motif of M. tuberculosis LAM, and on the functional characterization of two glycosyltransferases, MtxS and MtxT, responsible, respectively, for the production of decaprenyl-phospho-MTX (DP-MTX) and the transfer of MTX from DP-MTX to the mannoside caps of LAM. Collectively, our NMR spectroscopic and mass spectrometric analyses of mtxS and mtxT overexpressors and knockout mutants support a biosynthetic model wherein the conversion of 5'-methylthioadenosine, which is a ubiquitous byproduct of spermidine biosynthesis, into 5'-methylthioribose-1-phosphate precedes the formation of a 5'-methylthioribose nucleotide sugar, followed by the epimerization at C-3 of the ribose residue, and the transfer of MTX from the nucleotide sugar to decaprenyl-phosphate yielding the substrate for transfer onto LAM. The conservation of the MTX biosynthetic genes in a number of Actinomycetes suggests that this discrete glycosyl substituent may be more widespread in prokaryotes than originally thought.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| 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 source (direct Gemma or distilled Codex), 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".