Transcription of chick genes by mammalian RNA polymerase II in chick erythrocyte‐mammalian cell heterokaryons

The introduction of chick erythrocyte nuclei into mammalian cell cytoplasms results in their reactivation as evidenced by the de novo transcription of chick genes and the synthesis of both globin and constitutive proteins. In the present study, chick erythrocytes have been fused to L6 rat myoblasts and to alphaamanitin‐resistant variants of L6 to determine whether the chick or the mammalian RNA polymerase II was responsible for transcription of chick genes. Heterokaryons formed by fusing chick erythrocytes with alpha‐amanitin‐resistant L6 myoblasts synthesize both chick globin and chick constitutive proteins in the continued presence of 5 μg/ml alpha amanitin ten days postfusion. Both the synthesis of globin and other chick polypeptides occurs at levels comparable to those observed for untreated heterokaryons. Synthesis occurs under conditions in which insignificant chick RNA polymerase II activity can be detected irv wild‐type heterokaryons by autoradiography. These results demonstrate that RNA polymerase II is one of the mammalian proteins that is selectively taken up by the chick nucleus during reactivation in the presence of alpha amanitin. Furthermore, the mammalian RNA polymerase II alone can account for the transcription of both differentiation specific and constitutive genes in the chick nucleus.