Repression by TTK69 of GAGA-mediated Activation Occurs in the Absence of TTK69 Binding to DNA and Solely Requires the Contribution of the POZ/BTB Domain of TTK69

Abstract

tramtrack 69 (TTK69) is known to repress GAGA-mediated activation of the eve promoter in S2 cells. Here, we show that repression by TTK69 occurs in the absence of bona fide TTK69-binding sites on the template, indicating that it does not require the binding of TTK69 to DNA. Consistent with this interpretation, the POZ/BTB domain of TTK69, which does not bind DNA, is sufficient for repression. Moreover, a fusion protein in which the POZ/BTB domain of GAGA is replaced by that of TTK69 is not capable of activating the eve promoter but efficiently represses GAGA-dependent activation. Repression involves GAGA-TTK69 interaction because TTK69 is not capable of repressing basal transcription. Most probably, GAGA-TTK69 interaction occurs at the promoter because GAGA·TTK69 complexes are fully competent in binding DNA in vitro. Our results also show that repression by TTK69 of GAGA-dependent activation of the eve promoter is not mediated by any of the co-repressors known to interact with TTK69 (dMi2 or C-terminal binding protein) or by trichostatin A-sensitive histone deacetylases. Altogether, these observations strongly suggest that the binding of TTK69 prevents the interaction of GAGA with the transcription machinery and, therefore, compromises its activation potential.