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Acetylation of histones at DNA regulatory elements plays a critical role in transcriptional activation. Histones are also modified by other acyl moieties, including crotonyl, yet the mechanisms that govern acetylation versus crotonylation and the functional consequences of this "choice" remain unclear. We show that the coactivator p300 has both crotonyltransferase and acetyltransferase activities, and that p300-catalyzed histone crotonylation directly stimulates transcription to a greater degree than histone acetylation. Levels of histone crotonylation are regulated by the cellular concentration of crotonyl-CoA, which can be altered through genetic and environmental perturbations. In a cell-based model of transcriptional activation, increasing or decreasing the cellular concentration of crotonyl-CoA leads to enhanced or diminished gene expression, respectively, which correlates with the levels of histone crotonylation …