Question: Which is the methyl donor of histone methyltransferases?

Context:
While SAMe is a methyl donor, MTA is an inhibitor of methylation. SAMe can convert to MTA spontaneously, so the effect of exogenous SAMe may be mediated by MTA.
Based on known SET domain structures, the mutations likely affect either the lysine-substrate binding pocket, the binding site for the adenosylmethionine methyl donor, or a critical tyrosine predicted to interact with the substrate lysine epsilon-amino group
Also, S-adenosylhomocysteine or methyl donor deficiency inhibits RIZ1 and other H3 lysine 9 methylation activities.
Angiosperms synthesize S-methylmethionine (SMM) from methionine (Met) and S-adenosylmethionine (AdoMet) in a unique reaction catalyzed by Met S-methyltransferase (MMT). SMM serves as methyl donor for Met synthesis from homocysteine, catalyzed by homocysteine S-methyltransferase (HMT).
Plants synthesize S-methylmethionine (SMM) from S-adenosylmethionine (AdoMet), and methionine (Met) by a unique reaction and, like other organisms, use SMM as a methyl donor for Met synthesis from homocysteine (Hcy).
AtHMT-1 and -2 both utilize l-SMM or (S,S)-AdoMet as a methyl donor in vitro and have higher affinities for SMM.
When the serine residue is mutated to glutamic acid, which mimics the phosphorylated serine residue, the mutant CARM1 exhibits diminished ability to bind the methyl donor adenosylmethionine and diminished histone methylation activity.
S-adenosylmethionine (SAMe), the major methyl donor for DNA and histone methylation was fed with ethanol for 1month in order to modify the effects of ethanol on rat liver.
To investigate if tumor growth is dependent on the enzymatic activity of Ezh2, we developed a potent and selective small molecule inhibitor, EI1, which inhibits the enzymatic activity of Ezh2 through direct binding to the enzyme and competing with the methyl group donor S-Adenosyl methionine.
The enzyme was capable of transferring methyl groups to selected lysine residues in a substrate protein using S-adenosyl-l-methionine (SAM) as the methyl donor.
SAM is the main methyl group donor for methyltransferases to modify DNA, RNA, protein, metabolites, or phospholipid target substrates.
To control the enzymatic activity of vSET in vivo with an engineered S-adenosyl-l-methionine (SAM) analogue as methyl donor cofactor, we have carried out structure-guided design, synthesis, and characterization of orthogonal vSET methyltransferase mutant/SAM analogue pairs using a "bump-and-hole" strategy.
Protein lysine methyltransferases are important regulators of epigenetic signaling.
One of the well-studied PTMs, arginine methylation, is catalyzed by protein arginine methyltransferases (PRMTs) with SAM as the methyl donor.
While SAMe is a methyl donor, MTA is an inhibitor of methylation.
The purified cytochrome can act as a methyl acceptor for a methyltransferase activity in the cell extract that uses S-adenosylmethionine as methyl donor.
This enzyme utilizes S-adenosyl-L-methionine as the methyl donor.

Answer: The major methyl donor of histone methyltransferases (HMTs) is S-adenosyl-L–methionine (SAM, AdoMet).
Exact answer: S-adenosyl-L–methionine, SAM, AdoMet, S-Adenosylmethionine, adenosylmethionine, S-Adenosyl methionine