| Cell cycle transitions are highly regulated, including the transition during which cells exit the cell cycle prior to terminal differentiation. This regulation results in properly sized tissues and organs and prevents the formation of developmental defects and diseases such as cancer. Accordingly, understanding the mechanisms by which these events are coordinated will provide useful insights into both organismal development and disease states. Here I address unresolved questions about how cell cycle exit is achieved upon terminal differentiation in the Drosophila melangaster pupal eye and wing. An inclusive analysis examining the role of transcriptional regulation of cell cycle regulators in the establishment of exit determined that expression of negative regulators is not induced upon exit in the pupal wing. Moreover, inhibitory regulation of activating cell cycle regulators likely occurs post-transcriptionally. Prior genetic studies indicated that unknown factors contribute to the regulation of cell cycle exit; therefore I undertook a gain-of-function screen to identify new regulators of cell proliferation and cell cycle exit. I identified four genes not previously implicated in cell cycle regulation, two of which, cabut and mesr4 , cause delayed cell cycle exit when ectopically expressed. Further characterization described the Kruppel-like factor cabut as a negative regulator of cell cycle exit capable of inducing a single extra cell division in the differentiating wing or eye. This resembles the effect of ectopic expression of the E2F1 transcription factor or its target, Cyclin E. Surprisingly, I found that cabut does not act by increasing E2F or Cyclin E activity, but instead functions with E2F-like activity to regulate a large set of E2F target genes required for cell cycle progression. This work therefore contributes to the understanding of transcriptional events during normal cell cycle exit, identifies several new genes with potential cell cycle regulatory properties, and characterizes cabut as a new negative regulator of cell cycle exit with E2F-like activity. The results described here not only contribute to the understanding of cell cycle exit, but may be relevant to certain normal cell cycles or cells that cycle inappropriately, such as cancer cells. |
