Further experiments on the role of the nucleolus in the expression of structural genes.

When a chick erythrocyte nucleus is introduced into the cytoplasm of a mouse cell it undergoes reactivation and eventually determines the synthesis of chick-specific proteins in the cytoplasm of the heterokaryon. It has previously been shown that there is a close temporal correlation between the onset of synthesis of the chick-specific proteins and the appearance of nucleoli in the erythrocyte nuclei. We now show that the synthesis of chick-specific proteins in the heterokaryon decays if the nucleolus in the reactivated erythrocyte nucleus is irradiated with an ultraviolet microbeam. The synthesis of chick-specific proteins does not, however, decay if only one of two nucleoli in the erythrocyte nucleus or if extranucleolar areas of the nucleus are irradiated. These observations confirm that some function located at or near the nucleolus is essential for the full expression of structural genes.