Fig. 4. Mechanisms of interference with Ey functions through ectopic Pax proteins. Four mechanisms by which ectopic Pax proteins could interfere with the developmental program depending on Ey functions in eye-antennal discs are illustrated. In the first model, Pax proteins repress ey transcription either directly by blocking its enhancer (I), or indirectly by interfering with genes or their products required for ey activation (not shown). In the second (II) and third model (III), Pax proteins inhibit transcription of Ey target genes (X), activated in the wild type (wt) by Ey and a set of transcription factors (C), in a dominant negative manner. By contrast, in the fourth model (IV), ectopic Pax proteins do not interfere with transcription of ey or that of the targets of its product. Rather by altering the regulation of a set of target genes (Y), in combination with a set of transcription factors (D), ectopic Pax proteins activate a genetic program that interferes with the normal progression of the developmental pathway dependent on ey. While our results exclude models I-III and favor model IV of ‘developmental pathway interference’ activated by ectopic Pax proteins or other ectopic transcription factors, they do not rule out models I-III for few specific transcription factors not examined in this study.
