DOWN-REGULATION OF EXTRAMACROCHAETAE MESSENGER-RNA BY A DROSOPHILA NEURAL RNA-BINDING PROTEIN RBP9 WHICH IS HOMOLOGOUS TO HUMAN HU PROTEINS

Rbp9 is an RNA binding protein expressed mainly in the central nervous system of adult Drosophila melanogaster. Rbp9 shares a high degree of sequence similarity with human neural proteins referred to as Hu anti gens. Hu antigens bind to U-rich mRNA destabilizing elements with a hi gh affinity and, thus, have been implicated as regulators of mRNA stab ility. Using in vitro RNA binding assays, we found that Rbp9 binds str ongly to poly U sequences. We then employed a Selex system to identify a consensus Rbp9 binding site (UUUXUUUU). Information obtained from t he Selex results allowed the detection of two repeats of the Rbp9 cons ensus binding sequence in the 3' untranslated region of extramacrochae tae mRNA. UV crosslinking experiments demonstrated that Rbp9 interacts specifically with emc mRNA. The requirement of Rbp9 protein in the do wn regulation of emc mRNA was confirmed by northern (RNA) analysis, wh ich revealed that the level of emc mRNA increased 10-fold in rbp9 muta nt flies. Taken together with the in vitro RNA binding results, the ge netic evidence obtained strongly supports the hypothesis that Rbp9 fun ctions as a regulator of RNA stability.