Significant roles in RNA-binding for the amino-terminal regions of Drosophila Pumilio and Nanos

The Drosophila Pumilio (Pum) and Nanos (Nos) RNA-binding proteins govern abdominal segmentation in the early embryo, as well as a variety of other events during development. They bind together to a compound Nanos Response Element (NRE) present in thousands of maternal mRNAs in the ovary and embryo, including hunchback (hb) mRNA, thereby regulating poly-adenylation, translation, and stability. Many studies support a model in which mRNA recognition and effector recruitment are carried out by distinct regions of each protein. The well-ordered Pum and Nos RNA-binding domains (RBDs) are sufficient to specifically recognize NREs; the larger intrinsically disordered N-terminal regions (NTRs) of each protein have been thought to act by recruiting mRNA regulators. Here we use yeast interaction assays and experiments testing the regulation of hb mRNA in vivo to show that the NTRs play a significant role in recognition of the NRE, acting via two mechanisms. First, the Pum and Nos NTRs interact in trans to promote assembly of the Pum/Nos/NRE ternary complex. Second, the Pum NTR acts via an unknown mechanism in cis, modifying NRE recognition by its RBD. The ability of the NTR to alter binding to the NRE is conserved in human Pum2.