The reactivation of chicken erythrocyte nuclei in chick-mammalian heterokaryons resulted in the activation of chick globin gene expression. However, the level of chick globin synthesis was dependent on the mammalian parental cell type. The level of globin synthesis was high in chick erythrocyte-rat L6 myoblast heterokaryons but was 10-fold lower in chick erythrocyte-mouse A9 cell heterokaryons. Heterokaryons between chick erythrocytes and a hybrid cell line between L6 and A9 expressed chick globin at a level similar to that of A9 heterokaryons. Erythrocyte nuclei reactivated in murine NA neuroblastoma, 3T3, BHK and NRK cells, or in chicken fibroblasts expressed less than 5% chick globin compared with the chick erythrocyte-L6 myoblast heterokaryons. The amount of globin expressed in heterokaryons correlated with globin mRNA levels. Hemin increased beta globin synthesis two- to threefold in chick erythrocyte-NA neuroblastoma heterokaryons; however, total globin synthesis was still less than 10% that of L6 heterokaryons. Distinct from the variability in globin expression, chick erythrocyte heterokaryons synthesized chick constitutive polypeptides in similar amounts independent of the mammalian parental cell type. Approximately 40 constitutive chick polypeptides were detected in heterokaryons after immunopurification and two-dimensional gel electrophoresis. The pattern of synthesis of these polypeptides was similar in heterokaryons formed by fusing chicken erythrocytes with rat L6 myoblasts, hamster BHK cells, or mouse neuroblastoma cells. Three polypeptides synthesized by non-erythroid chicken cells but less so by embryonic erythrocytes were conspicuous in heterokaryons. Two abundant erythrocyte polypeptides were insignificant in non-erythroid chicken cells and in heterokaryons.

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