The role of acetylation in rDNA transcription

Treatment of NIH 3T3 cells with trichostatin A (TSA), an inhibitor of histo ne deacetylase (HDAC), resulted in a dose-dependent increase in transcripti on from a rDNA reporter and from endogenous rRNA genes. Chromatin immunopre cipitation using anti-acetyl-histone H4 antibodies demonstrated a direct ef fect of TSA on the acetylation state of the ribosomal chromatin. TSA did no t reverse inhibition of transcription from the rDNA reporter by retinoblast oma (Rb) protein, suggesting that the main mechanism by which Rb blocks rDN A transcription may not involve recruitment of deacetylases to rDNA chromat in. Overexpression of histone transacetylases p300, CBP and PCAF stimulated transcription in transfected NIH 3T3 cells. Recombinant p300, but not PCAF , stimulated rDNA transcription in vitro in the absence of nucleosomes, sug gesting that the stimulation of rDNA transcription by TSA might have a chro matin-independent component. We found that the rDNA transcription factor UB F was acetylated in vivo. Finally, we also demonstrated the nucleolar local ization of CBP. Our results suggest that the organization of ribosomal chro matin of higher eukaryotes is not static and that acetylation may be involv ed in affecting these dynamic changes directly through histone acetylation and/or through acetylation of UBF or one of the other components of rDNA tr anscription.