PU. 1, a hematopoietic Ets transcription factor, is required for development of the lymphoid and myeloid lineages. We have previously shown that PU. I functions as both a transcriptional activator and repressor through complex formation with CBP/p300 and HDAC1/mSin3A/MeCP2, respectively. To determine whether modification of PUA is responsible for switching its association between co-activators and co-repressors, we examined whether acetylation regulates the physical and functional activities of PU.1 PUA was acetylated in vivo and its repressor activity was reduced when the putative acetylation motifs in the Ets domain were mutated. The mutant cooperated with CBP similar to wild type PU. 1, but insufficiently with GATA- 1 and mSin3A. Whereas overexpression of wild type PU. 1 induced differentiation block, growth inhibition, and apoptotic cell death in MEL erythroleukemia cells as we reported previously, overexpression of the mutant-acetylation motif PU. 1 did not. Taken together, our data suggest that acetylation might regulate the biological functions of PUA in erythroid cells. (c) 2005 Elsevier Inc. All rights reserved.