Thermal denaturation studies of acetylated nucleosomes and oligonucleosomes

The thermal melting behaviors of control and acetylated mononucleosomes, dinucleosomes and trinucleosomes have been studied. Along each series of oligonucleosomes, the melting profiles change in a manner consistent with the increasing number of nucleosomes. For the control mononucleosome, the melting profile exhibits a premelting region at about 61-64 degrees C and a major cooperative transition at 75-77 degrees C. The melting profiles of the control dinucleosomes and trinucleosomes show a premelt at 61-62 degrees C (similar to that of the nucleosome core); an intermediate transition at 73-74 degrees C for the dinucleosome and at 76-77 degrees C for the trinucleosome and a major cooperative transition at 79-80 degrees C for the dinucleosome and at 81-82 degrees C for the trinucleosome. The major cooperative transition at the highest melting temperatures in the melting profiles of the mononucleosome, dinucleosome and trinucleosome comes from the melting of the central region of DNA in the nucleosome which complexed with the core histones; the premelt region is attributed to two DNA segments per nucleosome which flank this central DNA region and are free or weakly complexed with histones. The origin of the intermediate transition found for the dinucleosomes and trinucleosomes is not fully understood but probably results from the melting of DNA at the entry to and exit from the nucleosome and the linker DNA which are complexed with histones. A very similar pattern of behavior is observed for the acetylated oligonucleosomes. Direct comparison of the melting profiles of acetylated and control mononucleosomes, dinucleosomes and trinucleosomes show that the premelt region is unaffected by histone acetylation whereas the intermediate and major cooperative transitions for the acetylated oligonucleosomes are broader and occur consistently at lower temperatures than for the controls. These differences support proposals that the N-terminal regions of core histones interact within the nucleosome and on linker DNA.