Cis-regulatory elements of the mitotic regulator, string/Cdc25

Mitosis in most Drosophila cells is triggered by brief bursts of transcript ion of string (stg), a Cdc25-type phosphatase that activates the mitotic ki nase, Cdk1 (Cdc2), To understand how string transcription is regulated, we analyzed the expression of string-lacZ reporter genes covering similar to 4 0 kb of the string locus. We also tested protein coding fragments of the st ring locus of 6 kb to 31.6 kb for their ability to complement loss of strin g function in embryos and imaginal discs. A plethora of cis-acting elements spread over >30 kb control string transcription in different cells and tis sue types. Regulatory elements specific to subsets of epidermal cells, meso derm, trachea and nurse cells were identified, but the majority of the stri ng locus appears to be devoted to controlling cell proliferation during neu rogenesis. Consistent with this, compact promotor-proximal sequences are su fficient for string function during imaginal disc growth, but additional di stal elements are required for the development of neural structures in the eye, wing, leg and notum, We suggest that, during evolution, cell-type-spec ific control elements were acquired by a simple growth-regulated promoter a s a means of coordinating cell division with developmental processes, parti cularly neurogenesis.