The even skipped insulator Homie blocks Polycomb response element-mediated repression of the adjacent gene TER94. Miki Fujioka, James B Jaynes. Dept Biochem. & Mol. Biol, Thomas Jefferson Univ, Philadelphia, PA.
Previously, we identified a boundary region between even skipped (eve) and TER94 that has enhancer blocking activity, and promotes both long range enhancer-promoter communication and P-element homing. We call it Homie, for homing insulator of eve. Here, we show that one function of Homie in a native context is to prevent repression of TER94 by a Polycomb-response element (PRE) that is near the 3 end of the eve locus. When Homie is deleted, the normal ubiquitous expression of TER94 is repressed in both ovaries and embryos by the eve PRE. That is, TER94 is repressed when Homie is deleted, and expression returns when the PRE is also deleted. In embryos, the eve locus was shown by others to be a sharply delineated Polycomb (Pc) domain: both Pc binding and the associated histone modification H3K27me3 are seen throughout the eve locus, but not nearby in TER94. Consistent with Homie blocking the spread of PRE-dependent chromatin, we see spreading of the eve Pc domain into the TER94 region when Homie is deleted (or replaced by DNA), as evidenced by an increase in H3K27me3. Other known PREs substitute for the eve PRE to repress TER94 in the absence of Homie. Furthermore, most known insulators are capable of blocking PRE-dependent repression in this context. These studies reveal a novel function of both PREs and insulators during oogenesis and embryogenesis. When Homie is deleted, ubiquitous TER94 expression in embryos is replaced by expression in an eve pattern, suggesting that Homie also blocks the positive action of eve enhancers on the TER94 promoter. Intriguingly, when Homie is deleted, the eve enhancers that are located 3 of the eve transcription unit (between the eve promoter and Homie) show reduced activity, suggesting that Homie also facilitates the action of the 3 eve enhancers on its own promoter, while blocking their action on TER94.