RESUMEN
Active chromatin fraction from rat liver nuclei has been isolated under the lower ionic strength conditions, to prevent salt induced rearrangement and exchange of linker histone H1. Salt induced higher order folding in these oligonucleosomes was determined by sedimentation, viscosity, aggregation and circular dichroism studies. Sedimentation studies indicate that upon raising the ionic strength from 25 mM to 65 mM (mainly NaCl), active oligonucleosomes show intrafragmental interaction and formation of soluble oligomers. These oligomers disaggregate into unfolded monomers at 90 mM ionic strength. In contrast, oligonucleosomes from inactive genes show gradual increase in intrafragmental higher order folding without any interfragmental interaction on raising salt concentration. A much higher decrease in viscosity of active oligonucleosomes in comparison to bulk oligonucleosomes also support the above conclusion. However, on raising salt concentration above 100 mM NaCl, both the chromatin fractions are capable of forming insoluble aggregates. Decrease in the molar ellipticities of bulk oligonucleosomes at 273 and 282 nm is observed on raising ionic strength from 25 mM to 65 mM. A different pattern of this decrease is observed in case of active oligonucleosomes, indicating adaptation of a different type of salt induced secondary structure of DNA in these oligonucleosomes. Melting profile of DNA from active and bulk chromatin suggests that the base composition of both the chromatin fractions is same.