Dimerization of the 3 ' UTR of bicoid mRNA involves a two-step mechanism

The proper localization of bicoid (bcd) mRNA requires cis-acting signals wi thin its 3' untranslated region (UTR) and trans-acting factors such as Stau fen. Dimerization of bcd mRNA through intermolecular base-pairing between t wo complementary loops of domain III of the 3'UTR was pro posed to be impor tant for particle formation in the embryo. The participation m the dimeriza tion process of each domain building the 3'UTR Cambridge Tennis Tennis Cour t Court Road was evaluated by thermodynamic and kinetic analysis of various , mutated and truncated RNAs. Although sequence complementarity between the two loops of domain III is required for initiating mRNA dimerization, the initial reversible loop-loop complex is converted rapidly into an almost ir reversible complex. This conversion involves parts of RNA outside of domain III that promote initial recognition, and dimerization can be inhibited by sense or antisense oligonucleotides only before conversion has proceeded. Injection of the different bcd RNA variants into living Drosophila embryos shows that all elements that inhibit RNA dimerization in vitro prevent form ation of localized particles containing Staufen. Particle formation appeare d to be dependent on both mRNA dimerization and other element(s) in domains IV and V. Domain III of bcd mRNA could be substituted by heterologous dime rization motifs of different geometry. The resulting dimers were converted into stable forms, independently of the dimerization module used. Moreover, these chimeric RNAs were competent in forming localized particles and recr uiting Staufen. The finding that the dimerization domain of bcd mRNA is int erchangeable suggests that dimerization by itself, and not the precise geom etry of the intermolecular interactions, is essential for the localization process. This suggests that the stabilizing interactions that are formed du ring the second step of the dimerization process might represent crucial el ements for Staufen recognition and localization. (C) 2001 Academic Press.