Fig. 4. JNK is not the repressor and is dispensable for transcription activation by c-Jun. (A) wt-3T3 or jnk1^–/–, jnk2^–/– 3T3 cells were transiently transfected with a 5× UAS luciferase reporter and expression vectors for activator and competitor proteins as indicated. Fold activation compared with GAL–Jun_1–256 (activity set to 1, means ± SD of three independent experiments) is shown. (B) Phosphorylation of c-Jun by JNK weakens the interaction with the inhibitor. Left panel: wt-3T3 or jnk1^–/–, jnk2^–/– 3T3 cells were transiently transfected with a 5× UAS luciferase reporter and the GAL–Jun_1–256^Ala expression construct as activator, together with full-length c-Jun expression plasmid as competitor. Where indicated, ΔMEKK, was co-transfected. Activities were determined as in (A). The relative reporter gene activities (means ± SD of three independent experiments) are shown. The activity of GAL–Jun_1–256^Ala in the presence of c-Jun competitor was set to 100%. Right panel: model to explain the results shown in the graph on the left (for further details see text). (C) The δ-domain of c-Jun stabilizes the interaction with the repressor in the absence of JNK. wt-3T3 or jnk1^–/–, jnk2^–/–3T3 cells were transiently transfected with a 5× UAS luciferase reporter and expression constructs as indicated. Activities were determined as in (A). The % activation compared with that mediated by GAL–Jun + c-Jun, which was set to 100%, is shown (means ± SD of three independent experiments).