iminished al levels, suggesting that al is a Notch target gene. This would be the pre dicted relationship in a lateral inhibition system, where a Notch al positive feedback loop would amplify the Notch activity differences between neighboring cells. Two additional transcription factors that have been previously shown to be involved in leg morphogenesis were found to promote Notch signaling, Bonus, a homologue of the vertebrate TIF1beta transcriptional cofactor, and crooked legs, a zinc finger pro tein. Notch signaling is known to play an important role in Drosophila leg development, and the recovery of these two transcription factors as modifiers of Notch induced E m3 expression suggests that bon and croI may function to modulate Notch target gene output in the developing leg.
We also identified the Drosophila orthologues of two mammalian proto oncogenes kayak, and c Myb, as positive regulators of Notch signaling. Although a direct functional Entinostat link between these proteins and Notch signaling has not been described, kayak has been shown to interact genetically with Hairless and c Myb genetically interacts with bon, a novel Notch modifier described above. In addition, our data reveals a synergistic relationship between the positive regulator of Ras signaling, 14 3 3��, and Notch. Once again, the pro tein interaction network shows extensive contacts between 14 3 3�� and the chromatin machinery, suggest ing a mechanism for modulating Notch target transcrip tion through Su mediated chromatin modifications.
Interactions between Notch and oncogenic pathways are of particular interest, as the involvement of Notch in cancer biology and stem cell maintenance is becoming increasingly apparent. An unexpected Notch target transcription modifier identified in the screen is the Notch target gene Tram track. We found that targeting of ttk with dsRNA resulted in reduced Notch activity. In contrast, ttk expression itself has been shown to increase in response to ectopic Notch activity. The RNAi treatment data suggest that ttk may function in a posi tive feedback mechanism to promote Notch activated transcription and the network analysis suggests that this interaction may be mediated by a direct contact with Notch itself. Conclusions A complementary, genome wide RNAi approach has revealed a subset of factors that modulate Notch target transcription that may not have been found by tradi tional genetic approaches.
For instance, pleiotropic effects combined with non saturating mutagenesis may have obscured the detection of some components in tra ditional genetic screens. Several novel modifiers were identified in this RNAi transcription based screen, and these factors will be further investigated for their precise roles in the regulation of Notch signaling during devel opment. In addition, the interaction network of these factors suggests that many may work through contacts with chromatin machinery components that are in turn directed to Notch target promoters by the tran