Phosphorylation of transcription factors is regarded as a major mechanism to control their activity in regulation of gene expression. C/EBPp is a transcription factor that becomes activated after phosphorylation to induce genes involved in inflammation, acute-phase response, cytokine expression, cell growth, and differentiation. The chicken homolog NF-M collaborates with Myb and various kinase oncogenes in normal myeloid differentiation as well as in the leukemic transformation of myelomonocytic cells. Here, we examined the structure of NF-M and its mechanism of activation. We show that NF-M is a repressed transcription factor with concealed activation potential. Derepressed NF-M exhibits enhanced transcriptional efficacy in reporter assays. More importantly, NF-M activates resident chromatin-embedded, myelomonocyte-specific target genes, even in heterologous cell types such as fibroblasts or erythroblasts. We identified two regions within NF-M that act to repress trans-activation. Repression is abolished by deletion of these regions, activation of signal transduction kinases including v-erbB, polyoma middle T, ras and mil/raf, or point mutation of a critical phosphorylation site for MAP kinases. We provide evidence that phosphorylation plays a unique role to derepress rather than to enhance the trans-activation domain as a novel mechanism to regulate gene expression by NF-M/C/EBPp.

The receptor for the macrophage colony stimulating factor‐1 (CSF‐1R) is a transmembrane glycoprotein with intrinsic tyrosine kinase activity. CSF‐1 stimulation promotes the growth of cells of the macrophage lineage and of fibroblasts engineered to express CSF‐1R. We show that CSF‐1 stimulation resulted in activation of three Src family kinases, Src, Fyn and Yes. Concomitant with their activation, all three Src family kinases were found to associate with the ligand‐activated CSF‐1 receptor. These interactions were also demonstrated in SF9 insect cells co‐infected with viruses encoding the CSF‐1 receptor and Fyn, and the isolated SH2 domain of Fyn was capable of binding the CSF‐1R in vitro. Analysis of mutant CSF‐1Rs revealed that the ‘kinase insert’ (KI) domain of CSF‐1R was not required for interactions with Src family kinases, but that mutation of one of the receptor autophosphorylation sites, Tyr809, reduced both their binding and enzymatic activation. Because fibroblasts expressing this receptor mutant are unable to form colonies in semi‐solid medium or to grow in chemically defined medium in the presence of CSF‐1, the Src family kinases may play a physiological role in the mitogenic response to CSF‐1.

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