Proper deployment of Nanos protein at the posterior
of the Drosophila embryo, where it directs posterior
development, requires a combination of RNA localization
and translational controls. These controls ensure that
only the posteriorly-localized nanos mRNA is translated,
whereas unlocalized nanos mRNA is translationally
repressed. Here we describe cloning of the gene encoding
Smaug, an RNA-binding protein that interacts with the sequences,
SREs, in the nanos mRNA that mediate translational
repression. Using an in vitro translation assay, we demonstrate
that SRE-dependent repression occurs in extracts from early
stage embryos. Immunodepletion of Smaug from the extracts
eliminates repression, consistent with the notion that
Smaug is involved. Smaug is a novel gene and the existence
of potential mammalian Smaug homologs raises the possibility
that Smaug represents a new class of conserved translational
repressor.