Evidence of a differential organization of chromatin containing terminal or interstitial (TTAGGG)(n) repeats by in situ digestion with nucleases

(TTAGGG)(n) sequence repeats in human telomeres and in Chinese hamster inte rstitial centromeric areas were digested in situ with exonuclease III (ExoI II) and exonuclease Bal 31. Incubation with AluI was performed beforehand t o increase DNA breaks near telomere sequence areas. DNA removal at these sp ecific regions was quantified by digital image analysis of the fluorescence in situ hybridization signal produced by a telomeric probe. Exonuclease II I was 2.6 times more active in interstitial than in terminal telomeric sequ ence areas. Exonuclease Bal 31 was 2.3 times more effective in terminal tha n in interstitial telomeric sequence regions. These results support the hyp othesis that chromatin is differentially organized in both telomeric sequen ce areas, despite their similar DNA composition.