THE PUMILIO RNA-BINDING DOMAIN IS ALSO A TRANSLATIONAL REGULATOR

Posterior patterning in the Drosophila embryo requires the action of N anos (Nos) and Pumilio (Pum), which collaborate to regulate the transl ation of maternal hunchback (hb) mRNA. Previous work demonstrated that Pum recognizes sites in the 3' UTR of hb mRNA. In this report, we fir st define the RNA-binding domain of Pum and then show that residues es sential for translational repression are embedded within this domain. We also show that Nos and Pum can repress cap-independent translation from an internal ribosome entry site (IRES) in vivo, suggesting that t hey act downstream of the initial steps of normal, cap-dependent trans lation.