Fig. 9. Regulation of elements controlling bam transcription. The transcriptional silencer in the bam 5'-UTR is active in the cells in contact with cap cells, preventing bam expression. This effectively prevents GSCs from differentiating into cystoblasts and maintains them as a self-renewing population. Dpp signaling, and perhaps other signals delivered by diffusion and gap junction connections, are probably important positive regulators of silencer element-binding proteins (SEBPs). In the stem cell daughter, GSC maintenance signaling would be abrogated, the silencer would be inactive and bam would be produced under the control of the enhancer elements that drive transcription in germ cells. During the transition from active-to-inactive silencer, GSC daughters would be `pre-cystoblasts' until bam expression reached critical levels for full cystoblast differentiation. The switch from active-to-inactive silencer would also represent the biochemical mechanism for the asymmetry of the GSC division.

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