LexA chimeras reveal the function of Drosophila Fos as a context-dependenttranscriptional activator

The transcriptional activation potential of proteins can be assayed in chim eras containing a heterologous DNA-binding domain that mediates their recru itment to reporter genes. This approach has been widely used in yeast and i n transient mammalian cell assays. Here, we applied it to assay the transac tivation potential of proteins in transgenic Drosophila embryos. We found t hat a chimera between the DNA-binding bacterial LexA protein and the transa ctivation domain from yeast GAL4 behaved as a potent synthetic activator in all embryonic tissues. In contrast, a LexA chimera containing Drosophila F os (Dfos) required an unexpected degree of context to function as a transcr iptional activator. We provide evidence to suggest that this context is pro vided by Djun and Mad (a Drosophila Smad), and that these partner factors n eed to be activated by signaling from Jun N-terminal kinase and decapentapl egic, respectively Because Dfos behaves as an autonomous transcriptional ac tivator in more artificial assays systems. our data suggest that context-de pendence of transcription factors may be more prevalent than previously tho ught.