To separate causal effects of histone acetylation on chromatin accessibility and transcriptional output, we used integrated epigenomic and transcriptomic analyses following acute inhibition of major cellular lysine acetyltransferases P300 and CBP in hematological malignancies. We found that catalytic P300/CBP inhibition dynamically perturbs steady-state acetylation kinetics and suppresses oncogenic transcriptional networks in the absence of changes to chromatin accessibility. CRISPR-Cas9 screening identified NCOR1 and HDAC3 transcriptional co-repressors as the principal antagonists of P300/CBP by counteracting acetylation turnover kinetics. Finally, deacetylation of H3K27 provides nucleation sites for reciprocal methylation switching, a feature that can be exploited therapeutically by concomitant KDM6A and P300/CBP inhibition. Overall, this study indicates that the steady-state histone acetylation-methylation equilibrium functions as a molecular rheostat governing cellular transcription that is amenable to therapeutic exploitation as an anti-cancer regimen. Display omitted •Persistent P300/CBP KAT activity is required to maintain oncogenic gene expression•P300/CBP inhibition modulates transcription independently of DNA accessibility•Compensatory loss of NCoR/SMRT complexes overcomes catalytic P300/CBP inhibition•Histone methylation switching imparts stable transcriptional suppression Hogg et al. use multimodal genomics to explore the acute epigenetic and transcriptional effects of therapies targeting histone acetyltransferases P300/CBP in cancer. Catalytic P300/CBP inhibition selectively impairs expression of key oncogenic transcription factors, leading to robust anti-tumor responses, but remains inherently reversible prior to compensatory histone methylation switching.