RAS proteins exert their mitogenic and oncogenic effects through activation of downstream protein kinases¹. An important question is how Ras-generated signals reach the nucleus to activate down-stream target genes. AP-1, a heterodimeric complex of Jun and Fos proteins, which activates mitogen-inducible genes², is a major nuclear target of Ras³. 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) kinases⁴, which phosphorylate the transcription factor Elk-1/TCF^5,6. 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 loop². Ras- and ultra-violet-responsive protein kinases that phosphorylate c-Jun on ser-ine residues at positions 63 and 73 and stimulate its transcriptional activity have been identified⁹. These proline-directed kinases, termed JNKs, are novel MAP kinases¹⁰. 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 domain¹¹, suggesting that c-Fos may be similarly regulated. Here we show that Ras does indeed augment the transcriptional activity of c-Fos through phosphoryla-tion 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 kinases⁴ transmit Ras- and mitogen-generated signals to the transcriptional machinery.