Musashi and Seven in absentia downregulate Tramtrack through distinct mechanisms in Drosophila eye development

We have examined the roles played by the Drosophila neural RNA-binding prot ein Musashi (MSI) in eye development. MSI expression was observed in the nu clei of all photoreceptor cells (R1-R8). Although a msi loss-of-function mu tation resulted in only weak abnormalities in photoreceptor differentiation , we found that the msi eye phenotype was significantly enhanced in a seven in absentia (sina) background. sina is known to be involved in the degrada tion of the Tramtrack (TTK) protein, leading to the specification of the R7 fate. We demonstrated that MSI also functions to regulate TTK expression. The sina msi mutants showed significantly high ectopic expression of TTK69 and failure in the determination of the RI, R6, and R7 fates. Other photore ceptor cells also failed to differentiate with abnormalities occurring late in the differentiation process. These results suggest that MSI and SINA fu nction redundantly to downregulate TTK in developing photoreceptor cells. ( C) 1999 Elsevier Science Ireland Ltd. All rights reserved.