Suppression of SV40-promoted gene expression by differentiation of preadipose cells.

When a plasmid bearing the chloramphenicol acetyltransferase (CAT) gene under the control of an SV40 early promoter is introduced into preadipose or adipose cells of line 3T3-F442A, the promoter directs high levels of transient expression of CAT. However, when the plasmid is introduced into preadipose cells and the cells are then allowed to differentiate into adipose cells, the expression of the CAT gene is suppressed. In this process, the plasmid is not changed detectably in amount, topology, or state of methylation. Stably transformed preadipose cells bearing an integrated plasmid express the transferase, but if the cells are allowed to differentiate, the expression of the gene is similarly suppressed. The decline in CAT activity is associated with a decrease in the transcription rate of the gene. Transcription of a gene coding for neomycin phosphotransferase driven by the SV40 promoter is also greatly diminished by differentiation. Because suppression of CAT does not occur when the gene is under control of a retroviral long terminal repeat (LTR), a specific mechanism exists for the recognition and inactivation of the SV40 early promoter during differentiation.