Updated project metadata. In this project we present the identification of in vitro acetylation of Ub by the acetyltransferase p300. Free monomeric Ub or M1-linked linear Ub dimers (diUb) were incubated in vitro with p300 in presence or absence of acetyl-Coenzyme A (AcCoA). For identification of the acetylation status of the respective Ub variants parallel reaction monitoring (PRM), a targeted-MS approach, was used.