One of the most fundamental aspects of animal development is the control of the cell division cycle. Without this control there would be no coordination between cell proliferation and the formation of the gross anatomy of an organism. In my laboratory we are interested in understanding how the molecular processes that control pattern formation and cell fate decisions during Drosophila development influence the cell cycle. The ease with which we can analyze cell cycle behavior at a molecular level, and in the context of a whole animal, makes Drosophila an excellent experimental system for studying this problem. (For a movie of mitosis in the early fly embryo, click here.) We use genetic and biochemical approaches to examine how specific cell cycle regulators (e.g. E2F, cyclins, cdks) function, and then use this information to explore how their activity is influenced by different developmental programs.

We currently focus on the G1-S transition, a major control point of the cell cycle. A key component of the molecular machinery controlling the G1-S transition is the E2F transcription factor. E2F is part of a pathway including D-type cyclin/cdk and the pRB tumor suppressor protein that regulates entry into S phase in vertebrates. This pathway is evolutionarily conserved, and homologs of all its components have been identified in Drosophila. We have isolated mutations of Drosophila E2F, and used these to show that E2F is required for developmentally controlled S phase. This requirement is due to E2F's essential role in transcribing genes required for DNA replication, like cyclin E. In this way we suspect that the control of E2F activity provides a transcriptional mechanism linking developmental signals that control cell fate with cell cycle progress. Several projects in the lab are designed to test this hypothesis by further examining the E2F pathway and how it is regulated during development. We are also beginning to use genetic strategies to identify new genes that affect aspects of cell cycle regulation in addition to G1-S control.

Another project in the lab deals with one of the essential, conserved features of the G1-S transition in all animal cells: the synthesis of new histone mRNA. This mRNA synthesizes the histone proteins required to pack newly made DNA into chromatin. The expression of histone mRNA is tightly regulated during the the cell cycle: as cells progress from G1 to S-phase, histone mRNA levels increase 35-fold. These replication-dependent histone mRNAs are the only metazoan mRNAs that lack poly A tails, ending instead in a conserved 3’ stem-loop. Much of the cell-cycle regulation is post-transcriptional and is mediated by the 3' stem-loop. A 31 kDa stem-loop binding protein (SLBP) binds the 3' end of histone mRNA. SLBP is necessary for pre-mRNA processing and accompanies the histone mRNA to the cytoplasm where it is a component of the histone mRNP. The amount of SLBP is also regulated during the cell cycle, increasing 10-20 fold in late G1 and then decreasing during G2. SLBP levels are regulated at the level of translation as cells progress from G1 to S-phase and the protein is rapidly degraded as they progress into G2. Thus, regulation of SLBP may account for much of the cell cycle regulation of histone mRNA. In collaboration with the lab of Dr. Bill Marzluff, we have begun to genetically test this hypothesis using the genetic methodology available to fruit flies. We have cloned the Drosophila SLBP gene and have identified mutations of this locus. These mutations disrupt normal histone mRNA synthesis and block cell cycle progression, preventing normal development.