Reconstitution of a deoxyribonuclease I-sensitive structure on active genes.

Chicken erythrocyte nuclei have been labeled in the active regions of the chromosome by using the nick translation reaction. In this procedure, accessible areas of the genome are preferentially nicked by the action of pancreatic DNase I and subsequently labeled by using DNA polymerase I from Escherichia coli. These nuclei were employed as a substrate for studying the factors responsible for maintaining the special chromatin conformation of the overall population of active genes. Treatment of nuclei with 0.35 M NaCl resulted in the loss of DNase I sensitivity in the active genes, but this sensitivity could be restored when nuclei were reconstituted with the NaCl eluate. Further purification of the released factors revealed that the HMG (high-mobility group) proteins HMG-14 and HMG-17 are involved in maintaining the conformation of the active regions. These factors are not tissue specific and seem to be involved in the chromosomal structure of most of the active genes.