Selective functions for core promoter factors in neuroblast identity. Alexandre A. Neves, Robert N. Eisenman. Dept Basic Sci, Fred Hutchison Cancer Res Ctr, Seattle, WA.

   Development, homeostasis and repair of animal tissues require stem cells to balance self-renewal and differentiation. How stem cells maintain their identity through multiple rounds of division without differentiating is not well understood. Drosophila neuroblasts (NBs) have emerged as an ideal system to address this question because they can be readily identified and manipulated in vivo. NBs divide asymmetrically into a larger self-renewing daughter NB and a smaller, differentiating ganglion mother cell (GMC). Current models argue that asymmetric inheritance of cell fate determinants plays a major role in NB identity. However, the mutant phenotypes of such cell fate determinants suggest that multiple, redundant pathways are required for NB identity. To identify novel NB identity genes, we performed a focused RNAi screen using a worniuGAL4, UASGFP; UASDcr2 line to drive expression of RNAi transgenes in NBs and GMCs, and we used GFP expression as a proxy for NB fate. We focus here on genes encoding TATA -binding protein associated factors (Tafs) and TATA -binding protein related factor 2 (Trf2). In most cell types, Tafs bind to and function with Tbp, but not with its paralog Trf2. In contrast, we found that taf or trf2 knockdown (KD) brains, but not tbp KD brains, exhibited defects in NB morphology and homeostasis. The NB homeostasis function appears to be independent of survival as blocking apoptosis failed to suppress taf or trf2 phenotypes. Moreover, tafs and trf2 are required for the expression of the NB markers Asense and Cyclin E , but not Deadpan. We confirmed these results with taf MARCM clones. We further show that taf,tbp or trf2 knockdown NBs all express ectopic nuclear Prospero. Prospero is a transcription factor thought to be necessary and sufficient for GMC identity and is normally excluded from NB nuclei. However only tbp KD , but not taf or trf2 KD, suppresses supernumerary NBs that arise in a prospero background. Taken together, these results demonstrate novel, unexpected, and selective functions for core promoter factors in stem identity in vivo.