Rapid reprogramming of globin gene expression in transient heterokaryons.

Interspecific heterokaryons were formed by fusing adult mouse erythroleukemia (MEL) cells and human embryonic/fetal erythroid (K562) cells with each other, or with a variety of mouse and human nonerythroid cell types. Analysis of total cellular RNA isolated 24 hr after fusion revealed that normally inactive globin genes can be activated in these "transient" heterokaryons, in which the nuclei do not fuse. In general, the types of globin genes expressed in the donor erythroid cell are activated in the nucleus of the recipient cell. Therefore, erythroid cells contain transacting regulatory factors that are capable of activating the expression of globin genes in a stage- and tissue-specific manner. These observations also indicate that globin genes are not irreversibly repressed in differentiated cells and that their expression can be rapidly reprogrammed in the presence of the appropriate regulatory factors.