Fig. 9. (A) Three different mechanisms control repression of eve 2 in anterior regions of the embryo. The activity of Gt and at least one other factor (X) is required for repression very near the anterior border. Anterior to this domain, Slp1 and at least one other factor (Y) mediate eve 2 repression. At the anterior pole, Tor activity may downregulate the activity of Bcd, the primary activator of eve 2. (B) A model for Bcd coordination of eve 2 regulation. Bcd activates transcription of the eve 2 enhancer even at quite low concentrations. Bcd also is required for activation of the repressors of eve 2 (Gt and Slp1). We propose that these genes are positioned based on their decreased sensitivity to the Bcd gradient. At the anterior tip of the embryo, the activity of Bcd may be disrupted by the Tor phosphorylation cascade, which prevents activation of several Bcd target genes in this region.

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