Short-lived cytoplasmic regulators of gene expression in cell cybrids.

Somatic cell hybridization is a valuable tool for investigation the control of gene expression in eukaryotic cells. Studies of hybrid cells, heterokaryons, reconstructed cells and cybrids (cytoplasmic hybrids) have suggested that cytoplasmic factors may be involved in this regulatory process. Unfortunately, studies of this kind usually require that hybrid or modified cells be maintained for some time in a selective environment during which chromosomal losses or other changes may modify the genetic functions of the cells and thus vitiate conclusions about the mechanism of gene regulation. We report here the preparation of cybrids between enucleated mouse fibroblasts (Cl-1-D) and differentiated rat hepatoma cells (Fao) and the use of a combination of histological techniques to identify these modified cells early after fusion without the use of selective media. We found that albumin production in most cybrids was suppressed (extinguished) at 12-20 h after fusion but was restored by 48 h. These results suggest that there is a cytoplasmic factor in the fibroblast which exerts negative control over expression of the albumin gene, but which in the absence of the fibroblast nucleus, is not renewed and therefore short-lived.