A trans-acting factor represses the activity of the polyoma virus enhancer in undifferentiated embryonal carcinoma cells.

F9 undifferentiated embryonal carcinoma (EC) cells are resistant to infection by some papovaviruses and retroviruses. In the case of the polyoma virus the block in expression appears to be due to the inactivity of the transcriptional enhancer. We demonstrate here that the activity of the polyoma virus enhancer is repressed in undifferentiated F9 cells by a trans-acting factor which is not present in mouse fibroblasts.