Nucleo-cytoplasmic protein migration during the activation of chick erythrocyte nuclei in heterokaryons

The reactivation of the chick erythrocyte nucleus was studied after erythrocytes were induced to fuse with rat epithelial cells in the presence of Sendai virus. The chick nucleus swells, shows an increase in dry mass and protein content and resumes RNA synthesis. Nucleoplasmic antigens characteristic of the rat cell are found to migrate into the erythrocyte nucleus. The rate of uptake of these molecules, which are believed to be proteins, appears to be directly related to increases in nuclear size, ³H-uridine incorporation and RNA polymerase activity. The polymerase activity which increases during the first days after cell fusion is sensitive to α-amanitin but relatively resistant to actinomycin D. At later time points there is an increase in α-amanitin resistant polymerase activity which probably reflects the appearance of ribosomal RNA synthesis.When heterokaryons containing different proportions of rat: chick nuclei are compared, reactivation is found to proceed most rapidly in those containing a high rat: chick nuclear ratio. As the number of erythrocyte nuclei in heterokaryons increases, the rate of reactivation in the individual nuclei is progressively reduced suggesting that the erythrocyte nuclei compete with each other for macromolecules of specific importance for the activation process.