Expression of erythro-specific chick genes was studied in heterokaryons prepared by fusing chick erythrocytes (CE) with rat myoblasts. In this type of heterokaryon the inactive erythrocyte nucleus takes up nuclear proteins of myoblast origin and undergoes transcriptional reactivation. In order to study the stability of the genetic programming of the reactivated CE nucleus, chick gene expression was examined by analysis of RNA from the heterokaryons. Probes for several erythro- and chick-specific genes were used. The heterokaryons showed strong expression of the chick histone H5 and adult alpha-globin genes, while other genes, e.g. the transcription factor Eryf1 gene, normally expressed during erythroid differentiation, were not transcribed. Although the CE used were of the definitive lineage, the heterokaryons showed activation of the chick embryonic beta-globin gene, i.e. a gene normally expressed only in CE of the primitive lineage. We conclude that the reactivation of the mature CE nucleus in a rat cytoplasm resulted in a more immature erythroid gene expression pattern. The activation of the embryonic beta-globin gene indicated a switch of the lineage-specific gene expression pattern. This switch occurred in the absence of DNA replication. The strong expression of the globin and H5 genes in heterokaryons, in the absence of expression of the regulatory factor Eryf1, suggested the existence of Eryf1-independent regulatory mechanisms for erythroid gene expression in these cells.

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