The bHLH proteins Emc and Da control cell cycle progression through the transcriptional regulation of the Cdc25 phosphatase string, during Drosophila development. Irene Andrade-Zapata, Antonio Baonza. Centro de Biología Molecular Severo Ochoa, Madrid, Spain.

   The Helix-Loop-Helix (bHLH) family of transcription factors are key regulatory molecules that control multiple developmental processes, including cell differentiation and cell cycle control. They form heterodimers interacting through its HLH domain and are subdivided into groups, attending to their function, distribution, and DNA binding properties. Class V HLH proteins lack any basic domain, and as a consequence, heterodimers of class V proteins with other bHLH proteins are unable to bind DNA. Drosophila has a single class V protein, Extramacrochaetae (Emc), that is homologue to inhibitor of DNA binding (Id) proteins in vertebrates. It has been proposed that the function of this gene is necessary to maintain a proliferative state during organ development. Emc is known to form heterodimers with the class I protein Daughterless (Da). Recently, Bhattacharya and Baker (2011) have proposed that a cross-interacting regulatory network links expression of Da, which regulates its own expression, with expression of Emc, which antagonizes Da function. These authors suggest that most phenotypic effects, including cell proliferation defects, of mutating emc are due to the up-regulation of Da in emc mutant cells. However, the mechanisms by which this network regulates cell proliferation remain still unknown. In this work, we found that the reduction of emc or the over-expression of Da produces an accumulation of cells on G2 phase of the cell cycle. The main activator of the G2/M transition in eukaryotic cells is the string (Cdc25) phosphatase. We present evidences that indicate that the arrest in G2 phase of emc mutant cells and da over-expressing cells is a consequence of a reduction of string expression. Our results indicate that Da binds to string promoter, and function as a transcriptional repressor. We provide the first molecular mechanism to explain how the HLH proteins Emc and Da control cell proliferation during development.