ULTRABITHORAX AND ANTENNAPEDIA 5'-UNTRANSLATED REGIONS PROMOTE DEVELOPMENTALLY-REGULATED INTERNAL TRANSLATION INITIATION

The 5' untranslated regions (UTRs) of the Drosophila Ubx and Antp gene s were tested for their ability to promote cap-independent translation initiation. The Ubx and the Antp 5' UTR were inserted between the CAT and lacZ coding sequences in a dicistronic gene and tested for IRES a ctivity in transgenic Drosophila. Northern analysis of the mRNAs showe d the presence of the predicted full-length dicistronic mRNAs. High CA T activity was expressed from the first cistron from all of the dicist ronic constructs introduced into the fly genome. The dicistronic trans genic strains bearing the Ubx and Antp IRES elements expressed signifi cant levels of beta-galactosidase (beta GAL) from the second cistron w hereas little or no beta GAL was expressed in the controls lacking the IRESs. In situ analysis of beta GAL expression in the transgenic stra ins indicates that expression of the second cistron is spatially and t emporally regulated. Although the developmental patterns of expression directed by the Antp and Ubx IRESs overlap, they exhibit several diff erences indicating that these IRESs are not functionally equivalent.