FIG. 5.

Inactive topoisomerase I gene has SAF-A, p300, and acetylated histone H3 bound to its S/MAR element. (A) RT-PCR for the RNA expression of the topoisomerase I, p300, and GAPDH genes of serum-arrested T98G cells and after subsequent stimulation by serum for 6, 12, 18, 36, and 72 h, representing growing and subsequently (72 h) density-arrested cells. (B) ChIP assays with the serum-stimulated cells were analyzed with real-time PCR with primers within the topoisomerase I S/MAR element and in the topoisomerase I promoter. Depicted is the relative level of precipitated DNA with antibodies to SAF-A or p300 in comparison to a nonimmune control antibody. (C) ChIP assays with an antibody against acetylated histone H3 show high levels of histone H3 acetylation at S/MAR elements of topoisomerase I when the gene is inactive, in contrast to promoter and exon sequences that only show hyperacetylated histone H3 when the gene is expressed.