Previous studies have implicated acetylases and deacetylases in regulating the transcriptional activity of NF-kappa B. Here, we show that inhibitors of deacetylases such as trichostatin A (TSA) and sodium butyrate (NaBut) potentiated TNF-induced expression of several natural NF-kappa B-driven promoters. This transcriptional synergism observed between TNF and TSA (or NaBut) required intact kappa B sites in all promoters tested and was biologically relevant as demonstrated by RNase protection on two instances of endogenous NF-kappa B-regulated gene transcription. Importantly, TSA prolonged both TNF-induced DNA-binding activity and the presence of NF-kappa B in the nucleus. We showed that the p65 subunit of NF-kappa B was acetylated in vivo. However, this acetylation was weak, suggesting that other mechanisms could be implicated in the potentiated binding and transactivation activities of NF-kappa B after TNF plus TSA versus TNF treatment. Western blot and immunofluorescence confocal microscopy experiments revealed a delay in the cytoplasmic reappearance of the I kappa B alpha inhibitor that correlated temporally with the prolonged intranuclear binding and presence of NF-kappa B. This delay was due neither to a defect in I kappa B alpha mRNA production nor to a nuclear retention of I kappa B alpha but was rather due to a persistent proteasome-mediated degradation of I kappa B alpha. A prolongation of I kappa B kinase activity could explain, at least partially, the delayed I kappa B alpha cytoplasmic reappearance observed in presence of TNF plus TSA.