Fig. 3. Effect of histone H1 on accessibility for restriction enzymes of promoter DNA sequences in chromatin. (A) Cleavage by HinfI of wild-type MMTV (wtMMTV) and mutant MMTV (HRE/MMTVΔ) promoters in minichromosomes assembled in the presence or absence of histone H1. MMTV minichromosomes (200 ng of DNA) were assembled as described in Materials and methods. After assembly, the samples were digested at 26°C with 50 U of HinfI for 2, 4 or 8 min, and the DNA was purified and restricted with DraI (wtMMTV) or PvuII (HRE/MMTVΔ). The digestion products were analyzed by linear PCR. The positions of the HinfI uncleaved and cleaved fragments are indicated. The amount of cleavage as a percentage of total radioactivity is indicated below the lanes. (B) Cleavage by SacI of the wild-type MMTV promoter in minichromosomes assembled in the presence or absence of histone H1. MMTV minichromosomes (200 ng of DNA) were assembled in DREX, digested at 26°C with 50 U of SacI for 1, 2, 4 or 8 min, and the DNA was purified and restricted with DraI. The digestion products were analyzed by linear PCR. The positions of the SacI uncleaved and cleaved fragments are indicated. The amount of cleavage as a percentage of total radioactivity is indicated below the lanes.