DOT1L methylates H3K79 non-processively - so the three states are one distribution, not three signals
also called distributive H3K79 methylation · non-processive Dot1 · H3K79 methylation state redundancy
DOT1L does not hold onto a histone and add all three methyl groups in one go; it lets go between each one. That means the amount of mono-, di- and tri-methylation you see is just one number - how much enzyme activity happened there - split three ways, rather than three separate messages.
The same thing, for a scientist
Dot1 introduces multiple methyl groups by a **non-processive (distributive)** mechanism, unlike other characterised histone methyltransferases [E1]. The kinetic consequence stated by the authors is decisive for how this graph should reason: because the enzyme releases the substrate between additions, **the methylation states cannot be generated independently**, which implies functional redundancy between them. In yeast, Dot1-dependent silencing relied on global H3K79 methylation levels rather than on any one state, and H2B ubiquitination raised H3K79me3 by enhancing synthesis of *all* states [E1]. The mechanism is conserved across the family while the catalytic constants are not: yeast, trypanosome and human Dot1 enzymes share the distributive mechanism but have highly diverged rate constants and therefore very different steady-state patterns [E2]. Superimposed on the flux is one genuine state-boundary: H2B-K123 ubiquitination is dispensable for mono-methylation and required for every step beyond it, and it does this by regulating processivity rather than recruitment, since Dot1 still binds chromatin normally without it [E3]. The upshot is that **H3K79 methylation is a rheostat, not a code** - which the field has increasingly contested [E4] and which this graph should hold as an open conflict rather than a settled model.
Why this entry corrects a premise this project was working from
The dispatch brief for this beat stated that the H3K79 marks are "deposited processively". The primary literature says the opposite, and says it in the title: Nonprocessive methylation by Dot1 leads to functional redundancy of histone H3K79 methylation states 1. The distinction is not semantic. A processive mechanism would let the cell choose a state; a distributive one means the state distribution is a readout of enzyme-time at that nucleosome.
This matters for three downstream claims the graph makes or could make:
- State-specific function. Under distributive kinetics, state-specific readers and state-specific demethylases have less to do, and the original authors said so explicitly - multiple methylation "is expected to limit the possibilities for regulation by putative demethylases or binding proteins" 1.
- Biomarkers. If me1, me2 and me3 are three points on one distribution, then a pharmacodynamic assay reading one state is reading a monotone but non-linear function of enzyme flux, and the three states will not move proportionally.
- Inhibitor pharmacology. A distributive enzyme's output depends on the fraction of time it is uninhibited at a locus, which makes exposure duration, not peak concentration, the variable that sets the mark. That is a direct argument about the pinometostat schedule and it is made in
hyp-pinometostat-wrong-modality.
The one real state boundary: H2B ubiquitination
Distributivity does not mean there are no discontinuities. There is exactly one well-established one. Ubiquitylation of H2B-K123 (yeast; K120 in human) is dispensable for mono-methylation of H3K79 and required for the transition to di- and tri-methylation, and Dot1 binding to chromatin is normal in its absence 3. So the H2Bub-negative state of chromatin supports me1 only.
This is the reason mark-h3k79me1 is biologically separable from mark-h3k79me2/me3 even under a rheostat model, and it is the mechanistic joint between this beat and the rest of Q3b's folder: the H2B ubiquitination machinery (gene-rnf20, gene-rnf40, gene-ube2a, gene-wac) is, on this model, the switch that decides whether DOT1L's rheostat can go past one.
The modern yeast zoning result is consistent with that half and goes further: Rad6 loss abolishes the H3K79me3 zone outright and converts it to an H3K79me1-enriched region 4.
The conflict, left open
The redundancy model 1 and the zoning model 4 make incompatible predictions and this graph holds both. The redundancy model predicts that anything achievable by one state is achievable by another at sufficient global level; the zoning model predicts three gene classes each dominated by one state, stable across transcriptional reprogramming, with opposite transcriptional consequences.
What would settle it in a way this project could use: measure the three states at KMT2A-fusion target loci with calibrated internal-standard ChIP across a graded DOT1L-inhibitor dose series, and ask whether the state ratios move together (redundancy) or whether some loci hold a state ratio as flux falls (zoning). That experiment is written as exp-h3k79-state-resolved-icechip-dose-series.
| mechanism class | enzyme kinetics / chromatin mark deposition |
|---|---|
| enzyme | gene-dot1l |
| substrate | nucleosomal histone H3 K79 (lateral surface) |
| kinetic mechanism | distributive (non-processive) - substrate released between successive methyl additions |
| state boundary | H2B-K120/K123 monoubiquitination is required for every step beyond mono-methylation and dispensable for mono-methylation itself |
| brief premise corrected | The dispatch brief stated the marks are 'deposited processively'; the primary literature (PMID 18511943) establishes the opposite. |
conserved across
[ "S. cerevisiae Dot1", "T. brucei TbDot1A/TbDot1B (H3K76)", "human DOT1L" ]
competing models
[
{
"model": "functional redundancy / rheostat",
"source_pmid": "18511943",
"prediction": "state ratios move together with enzyme flux; global level is what matters"
},
{
"model": "state-specific zones",
"source_pmid": "41925229",
"prediction": "loci hold state ratios that are stable across transcriptional reprogramming and have opposite transcriptional consequences"
}
]- If methylation state is a rheostat set by enzyme-time, then intermittent dosing of a reversible inhibitor lets the mark re-accumulate between doses in a way that a fixed-occupancy model of target engagement does not predict.
- Any intervention aimed at the H2B ubiquitination machinery to cap H3K79 methylation at me1 will also cap H3K4 methylation by the same crosstalk, so the selectivity that makes it attractive is not available.
- At a locus where an MLL fusion tethers DOT1L, does the local state distribution still obey distributive kinetics, or does tethering create effective processivity by raising the rebinding rate far above the release rate?
- Is the redundancy-versus-zoning conflict a species difference, an assay difference or an antibody difference? No study has run both designs in the same system.
- Does H2Bub occupancy at KMT2A-fusion target genes set a ceiling on the reachable methylation state there, and is that ceiling heterogeneous across the fusion's targets?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Dot1, in contrast to other known histone methyltransferases, introduces multiple methyl groups via a non-processive mechanism; the kinetic mechanism implies that the H3K79 methylation states cannot be generated independently, suggesting functional redundancy, and Dot1-dependent gene silencing in yeast relied on global H3K79 methylation levels rather than on one specific methylation state. H2B ubiquitination affects H3K79 trimethylation by enhancing synthesis of all H3K79 methylation states.
Nonprocessive methylation by Dot1 leads to functional redundancy of histone H3K79 methylation states. · Frederiks F, Tzouros M, Oudgenoeg G, et al. · Nat Struct Mol Biol 2008What this evidence cannot show
Abstract read from the eutils record; full text not read. Yeast Dot1 and yeast silencing; the enzyme mechanism is shown for Dot1 and the functional redundancy is shown for one phenotype (telomeric/silencing) in one organism. 'Functional redundancy' is inferred from the kinetic mechanism plus a silencing assay, not demonstrated for any mammalian phenotype and certainly not for a leukaemic one. The H2B-ubiquitination statement is described by the authors as a suggestion.
Evidence that argues against this (1)
- The 2026 zoning result is the direct challenge: three distinct gene groups each predominantly marked by one H3K79 methylation state, stable during transcriptional reprogramming, and with opposite transcriptional associations - Dot1-activated genes enriched for me3, Dot1-repressed genes for me1. If that holds, state distribution is not merely a readout of flux. Park NH et al. 2026
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
The conserved Dot1 family establishes its H3K79 (H3K76 in trypanosomes) methylation pattern via a distributive mechanism shared between the yeast, Trypanosoma brucei and human enzymes, which nonetheless have highly distinct catalytic properties and generate very divergent steady-state methylation patterns.
Dot1 histone methyltransferases share a distributive mechanism but have highly diverged catalytic properties. · Stulemeijer IJ, De Vos D, van Harten K, et al. · Sci Rep 2015What this evidence cannot show
Abstract read from the eutils record; full text not read. Heterologous enzymes expressed in budding yeast at varying levels, with computational simulation used to infer catalytic properties; human DOT1L's constants are therefore inferred in yeast chromatin, not measured on human nucleosomes. Bulk methylation distributions, no locus resolution.
Evidence that argues against this (1)
- Bulk distributive kinetics cannot predict the local state distribution where an oncogenic fusion tethers the enzyme. In MLL-AF9 cells, graded weakening of the DOT1L-AF9 interaction produced DIFFERENTIAL rather than proportional loss of me2 and me3 at target genes, which is a locus-level behaviour a bulk model does not capture. Kuntimaddi A et al. 2015
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Ubiquitylation of H2B-K123 is dispensable for monomethylation of H3-K79 and H3-K4 but is required for the transition from monomethylation to subsequent methylation states; Dot1 binds chromatin normally without it, so H2B ubiquitylation regulates the processive activity of the methyltransferase rather than its recruitment.
Histone H2B ubiquitylation controls processive methylation but not monomethylation by Dot1 and Set1. · Shahbazian MD, Zhang K, Grunstein M · Mol Cell 2005What this evidence cannot show
Abstract read from the eutils record; full text not read. Saccharomyces cerevisiae, H2B-K123. Bulk ChIP plus mass spectrometry. The paper's own terminology ('processive activity') sits awkwardly beside the later demonstration that the mechanism is non-processive; the operational content - H2Bub is needed for steps beyond mono - is what this entry relies on, not the word.
Evidence that argues against this (1)
- The human structural work shows the H2Bub-DOT1L relationship is not only about methylation flux: DOT1L reads ubiquitin through a C-terminal hydrophobic helix and the H2A/H2B acidic patch through an arginine anchor, and H2Bub enhances a NON-catalytic nucleosome-destabilising activity of DOT1L. Treating H2Bub purely as a processivity licence misses a second, catalysis-independent output. Jang S et al. 2019
- E4moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
ChIP-seq mapping of the three H3K79 methylation states across the yeast genome identifies three state-specific methylation zones that remain largely stable during transcriptional reprogramming; loss of Rad6 causes complete loss of the H3K79me3 zone and its conversion to an H3K79me1-enriched region, and loss of H4K16 acetylation weakly disrupts the H3K79me1 zone.
Trans-histone crosstalk establishes distinct H3K79 methylation zones with differential transcriptional functions. · Park NH, Kim HR, Kim HY, et al. · Nucleic Acids Res 2026What this evidence cannot show
Abstract read from the eutils record; full text not read. Yeast. Published 2026, not independently replicated, and the entire zone model depends on state-specific antibody discrimination between marks that co-occur on the same residue - the abstract reports no peptide-array specificity control. The authors hedge the mechanism themselves ('may be established and/or maintained via').
Evidence that argues against this (1)
- The kinetic mechanism predicts exactly the correlation structure that would make apparent 'zones' out of a single flux variable: high-flux loci accumulate me3 and low-flux loci stall at me1, producing gene groups dominated by one state without any state-specific regulation. The stability of zones across transcriptional reprogramming is the observation that argues against this, and it is the part most in need of replication. Frederiks F et al. 2008
distributive kinetics set the me1 share of the distribution
distributive kinetics set the me2 share of the distribution
distributive kinetics make me3 the flux-limited state
- depends on →The trans-histone crosstalk H2Bub → DOT1L → H3K79me: modulatory for me1, effectively required for me2/me3
H2Bub is the one real state boundary in an otherwise continuous rheostat
the enzyme whose kinetics this entry describes
- ← related toNobody has convincingly found the H3K79 eraser - and that constraint sets the clock on every DOT1L inhibitor
co-occurring states are a prior against state-specific erasure
rival model of what sets the me1:me2:me3 distribution
do state ratios move together (redundancy) or hold per locus (zoning)