Almost every Tagmentation library prep protocol has a reaction buffer which contains a high concentration of DMF but I can't find a reason why it's included in the reaction buffer for the Tn5 transposase. The closest I've found is a protocol which claims it's a crowding agent but that claim is unsourced, and I think might be due to a misunderstanding of a statement in DOI: 10.1101/gr.177881.114 where the authors had to replace DMF with a crowding agent in situations where the starting DNA concentration was very low. (Also, DMF has a much lower molecular weight than common crowding agents like PEG.)
Anyone know why it's there? The only thing I've seen DMF used for in biology is as a solvent for reagents with poor water solubility like X-gal. It's really unusual to see it in an enzymatic buffer, especially at concentrations exceeding 10%.
I stumbled on your question by researching alternatives to DMF during tagmentation. I may have a partial answer to your question.
In a Steve Henikoff's paper the team states in their protocol that "10% 1,6-hexanediol or N,N-dimethylformamide compete for hydrophobic interactions and result in improved tethered Tn5 accessibility and library yield at the expense of slightly increased background". Article Simplified Epigenome Profiling Using Antibody-tethered Tagmentation
Therefore improving Tn5 accessibility through hydrophopbic interactions is the main reason they state for its use in CUT&TAG. Now, why specifically DMF and not, say, any other similar polar aprotic solvent is not clear. That is indeed a good question to look into! Good luck!
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