Reactivation of DNA replication in erythrocyte nuclei by Xenopus egg extract involves energy-dependent chromatin decondensation and changes in histone phosphorylation

Reactivation of chicken erythrocyte nuclei for DNA replication in Xenopus egg extracts involves two phases of chromatin remodelling: a fast decondensation leading to a small volume increase and chromatin dispersion occurring within a few minutes (termed stage I decondensation), followed by a slower membrane-dependent decondensation and enlargement of up to 40-fold from the initial volume (stage II decondensation). Chromatin decondensation as measured by nuclear swelling and micrococcal nuclease digestion required ATP. We observed a characteristic change in the phosphorylation pattern of erythrocyte proteins upon incubation in egg extract. While histones H5, H2A, and H4 became selectively phosphorylated during decondensation, the phosphorylation of histone H3 and of several nonhistone proteins was prevented. Furthermore, histone H5 was selectively released from erythrocyte nuclei in an energy-dependent reaction. These molecular changes already occurred during stage I decondensation and they persisted during stage II decondensation. DNA replication was confined to nuclei of stage II decondensation which incorporated lamin LIII from the egg extract. These results show that initiation of DNA replication in chicken erythrocytes requires in addition to ATP-dependent chromatin remodelling (stage I), further changes in chromatin structure that correlates with lamin LIII incorporation, and stage II decondensation.