Differential stability of the higher order structure of chromatin associated with genes having different transcriptional activity.

We have investigated the stability of the higher order structure of chromatin associated to genes which display a different transcriptional activity in adult rat live. Nuclei were digested with micrococcal nuclease and chromatin was fractionated by sedimentation in sucrose gradients. Specific DNA sequences were revealed by dot-blotting. In conditions of physiological ionic strength the distribution of the inactive gamma-casein gene sequences is similar than the bulk of chromatin. In the same conditions the relative content of the albumin gene, highly expressed in adult rat liver, revealed an enhanced instability of the chromatin superstructure. The distribution of the potentially active but silent alpha-fetoprotein sequences in adult liver showed an intermediate unfolding of its chromatin superstructure. These distinct behavior was not observed in non-physiological ionic strength conditions. Our results suggest that distinct folding of the local higher order structure of chromatin actually occurs in the region of active, potentially active and inactive genes.