Chapter 5 Chemical and Structural Changes Within Chick Erythrocyte Nuclei Introduced Into Mammalian Cells by Cell Fusion

Abstract

This chapter discusses the recent studies on chick erythrocyte nucleus, as it undergoes the reactivation reaction. The chapter describes the macromolecular and structural changes within the erythrocyte nucleus. The chick erythrocyte is a differentiated cell and its nucleus is very low in activity related to macromolecular synthesis. The inactivation occurs gradually during erythropoiesis and is paralleled by the changes in nuclear composition and condensation of the chromatin. When a chick erythrocyte nucleus is introduced, by cell fusion into the cytoplasm of a mammalian cell, the first event is redistribution of the cellular p301 of mammalian nuclear protein. The chapter discusses the fate of chick nuclear material and the changes in protein composition within the reactivating chick nuclei. In addition, the changes in nucleic acid metabolism (RNA and DNA synthesis) are also discussed in this chapter. The cause-effect relationships suggest that one event during the reactivation may depend on the other independent events, thus leading to the reactivation of the chick erythrocyte genome. The general problems in analyzing the experiments are the time sequence of events, competition for rate-limiting factors, and uncoupling reactions. After the reactivation of chick erythrocyte nuclei in mammalian tissue culture cells, the chick nucleus acquires a broad spectrum of the mammalian proteins, because at the early stages of reactivation the mammalian nucleus is the only one specifying the synthesis of nuclear proteins in the heterokaryon. Among the mammalian nucleospecific proteins that accumulate, some proteins should be considered as signals of direct regulatory importance. Further analysis of the reactivation reaction should lead to an identification of such signals.