Active chromatin of oocytes injected with somatic cell nuclei or cloned DNA

Abstract

It has been previously found that somatic cell nuclei injected into Xenopus oocytes undergo major changes in structure and greatly increase their rate of RNA synthesis. We show here that these changes are not accompanied by a general displacement of histones or of the non-histone chromosomal proteins known as HMGs, when complete nuclei or in vitro assembled DNA-protein complexes are injected and analyzed. Furthermore injected nuclei do not undergo any general change from the transcriptionally inactive conformation to an HMG-binding “active” conformation. Chromatin assembled by salt dialysis with a 145-base pair spacing of nucleosomes is not changed to the typical 200-base pair spacing. We conclude that the changes in gene activity undergone by somatic nuclei in oocytes affect a specific minority of genes and are not achieved by gross chromosomal changes. We have also analyzed the nucleo-protein structures formed from purified DNA injected into oocytes. Although the majority of these are known from previous work to be transciptionally inactive, we find that they are in an “active” HMG-binding conformation. This is attributable to the undermethylated state of the injected cloned DNA, since the same DNA after methylation in vitro is assembled in oocytes into a structure which does not bind HMGs. The HMG-binding structures formed from injected DNA contain a low level of HMG proteins, which we find to be present in unusually small amounts in Xenopus oocytes and eggs.