Ternary complex factors (TCFs) bind to the serum response element in the c-fos promoter and mediate its activation by many extracellular stimuli. Some of these stimuli activate the ERK subclass of mitogen-activated protein kinases (MAPKs) that target the TCF Sap-1a. We show that Sap-1a is also phosphorylated by the stress-activated JNK subclass of MAPKs leading to stimulation of both c-fos serum response element and E74-site-dependent transcription in RK13 cells. Several JNK-1 phosphorylation sites were mapped within Sap-1a, and mutation of these sites affected the transactivation mediated by Sap-1a and JNK-1. The impact of these phosphorylation sites varied at different promoters and was dependent on whether Sap-1a was stimulated by ERK-1 or JNK-1. Additionally, a comparison of Sap-1a with another TCF, Elk-1, revealed that these proteins behaved differently to stimulation by ERK-1 and JNK-1. Furthermore, activation of Sap-1a by JNK-1 was inhibited by the p38(MAPK) in RK13 cells, possibly by competition for a common upstream activator. Altogether, our data suggest that Sap-1a plays an important role in the nuclear response elicited by cellular stress.