Establishment and Maintenance

The principle that a regulatory decision is first made in an establishment phase, then ratified and made permanent in a maintenance phase, is observed in a variety of systems. Perhaps the best studied case is that of Sxl in Drosophila. In the establishment phase, Sxl is expressed from the Pe promoter in females but not in males (17). Translation of the Pe transcript makes active Sxl protein. In the maintenance phase, the Sxl gene is expressed from a different promoter, the Pm promoter, both in males and in females; however, the following positive autoregulatory loop ensures that Sxl protein is made only in females. The Sxl protein is required for proper splicing of the latter mRNA, and only in its presence can the Pm transcript be spliced properly. Accordingly, in females the presence of Sxl protein made in the establishment phase allows more Sxl protein to be made in the maintenance phase; in males, the Pm transcript cannot be properly spliced, and no Sxl protein is made. As another example, Drosophila proteins in the Polycomb group act to maintain patterns of gene expression established by other mechanisms, doing so by maintaining repressive chromatin structures (4).

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