RAS proteins exert their mitogenic and oncogenic effects through activation of downstream protein kinases(1). An important question is how Ras-generated signals reach the nucleus to activate downstream target genes. AP-1, a heterodimeric complex of Jun and Fos proteins, which activates mitogen-inducible genes(2), is a major nuclear target of Ras(3). Ras can stimulate AP-1 activity by inducing c-fos transcription(2,3), a process which is probably mediated by the ERK1 and -2 mitogen-activated protein (MAP) kinases(4), which phosphorylate the transcription factor Elk-1/TCF5,6. Besides inducing transcription from fos and jun genes, mitogens and Ras proteins enhance AP-1 activity through phosphorylation of c-Jun(7,8). Phosphorylation of the c-Jun activation domain leads to c-jun induction through an autoregulatory loop(2). Ras- and ultraviolet-responsive protein kinases that phosphorylate c-Jun on serine residues at positions 63 and 73 and stimulate its transcriptional activity have been identified(9). These proline-directed kinases, termed JNKs, are novel MAP kinases(10). It is not clear, however, whether c-Jun is the only recipient and JNK the only transducer of the Ras signal to AP-1 proteins, A short sequence surrounding the major JNK phosphorylation site of c-Jun is conserved in c-Fos and is part of its activation domain(11), suggesting that c-Fos may be similarly regulated. Here we show that Ras does indeed augment the transcriptional activity of c-Fos through phosphorylation at Thr 232, the homologue of Ser 73 of c-Jun. However, this is mediated by a novel Ras- and mitogen-responsive proline-directed protein kinase that is different from JNKs and ERKs. Therefore, at least three types of proline-directed kinases(4) transmit Ras- and mitogen-generated signals to the transcriptional machinery.