C-FOS TRANSCRIPTIONAL ACTIVATION AND REPRESSION CORRELATE TEMPORALLY WITH THE PHOSPHORYLATION STATUS OF TCF

EGF-induction of human astrocytoma and A431 cells leads to c-fos trans criptional activation and then repression. This could be correlated wi th changes in the DNA binding characteristics of the c-fos regulatory protein ternary complex factor (TCF) present in nuclear extracts from these cells. Band shifts showed the appearance of induction-related sl owly migrating protein-DNA complexes, detected as ternary complexes on the c-fos SRE using a truncated SRF molecule and by direct binding to the Drosophila E74 Ets-protein recognition sequence. By several crite ria both types of complexes represented TCF. The appearance of the slo w ternary and direct complexes correlated with c-fos transcriptional a ctivation, and their disappearance coincided with the ensuing c-fos sh ut-off. Blocking c-fos transcriptional repression with the phosphatase inhibitor okadaic acid led to their continued presence. They were sen sitive to protein phosphatase 2A but not 1alpha, and similar slow comp lexes were formed by partially purified p62TCF phosphorylated by a cop urifying kinase activity. Thus the phosphorylation state of TCF correl ated strongly with c-fos promoter activity. Since ternary complex form ation mediated by full-sized SRF was only slightly affected under comp arable conditions, we propose a model for c-fos regulation involving m odification of constitutively bound TCF.