High‐level Expression of Human c‐jun Gene Causes Cellular Transformation of Chicken Embryo Fibroblasts

To analyze the transforming activity of c‐jun, a Rous sarcoma virus (RSV) variant that carries human c‐jun Instead of v‐src (JH1) was constructed. After infection onto chicken embryo fibroblasts (CEF), JH1 formed foci with a titer comparable to that of wild‐type RSV, and the infected cells grew in soft agar, indicating that the human c‐jun gene has transforming potential, like the v‐jun gene. The expression of Fra‐2, one of the recently isolated Fos‐related antigens, hut not Fos was detected in both JHl‐infected CEF and CEF infected with the control retrovirus vector (DS3). Gel shift analysis using nuclear extracts from DS3‐infected CEF revealed that the Fra‐2/Jun complex contributes to the basal level of AP‐1 DNA binding activity. A similar activity was detected in JHl‐infected CEF, but these cells have an additional AP‐1 binding activity derived from Jun homodimers that seems to play important roles in the cellular transformation.