UltrabithoraxandAntennapedia5′ Untranslated Regions Promote Developmentally Regulated Internal Translation Initiation

Abstract

The 5 untranslated regions (UTRs) of the Drosophila Ubx and Antp genes 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 activity in transgenic Drosophila. Northern analysis of the mRNAs showed the presence of the predicted full-length dicistronic mRNAs. High CAT activity was expressed from the first cistron from all of the dicistronic constructs introduced into the fly genome. The dicistronic transgenic strains bearing the Ubx and Antp IRES elements expressed significant levels of ␤-galactosidase (␤GAL) from the second cistron whereas little or no ␤GAL was expressed in the controls lacking the IRESs. In situ analysis of ␤GAL expression in the transgenic strains indicates that expression of the second cistron is spatially and temporally regulated. Although the developmental patterns of expression directed by the Antp and Ubx IRESs overlap, they exhibit several differences indicating that these IRESs are not functionally equivalent.The cap-independent internal initiation model was proposed initially to explain the mechanism of translation initiation of picornavirus mRNAs (2, 16-18, 25, 30, 35). The unique structural characteristics of picornoviral mRNAs including the absence of a cap structure at the 5Ј end, the presence of extraordinarily long and structured 5Ј untranslated regions (UTRs), and the presence of multiple upstream AUGs are incompatible with the cap-dependent scanning model purported to explain the translation of all cellular mRNAs. Upon infection of mammalian cells with poliovirus, the cap binding complex (eIF-

Two isoforms of the Notch antagonist Hairless are produced by differential translation initiation

“…As cell-cycle-dependent differential stability of the two H isoforms seems not very likely, we favor the idea that H p120 is translated during mitotic phases when H p150 cannot be produced. In Drosophila, other functional IRES sequences have been identified before (26), but not with regard to cellcycle-regulated translation. It will be interesting to see whether all of these mRNAs show a biphasic translation pattern also in Drosophila.…”

Transcriptional and Translational Regulation of β-Cell Differentiation Factor Nkx6.1

“…By using this mechanism, the translation of a number of mammalian growth factor RNAs is specifically regulated during differentiation or cell growth (19 -22). Furthermore, during Drosophila embryonic development, the 5Ј-UTRs of mRNAs encoding homeodomain transcription factors Antp and Ubx are known to regulate protein expression in a spatiotemporal manner, although there are no reports of the existence of IRESs in any mammalian homeodomain transcription factor genes (23,24).…”

Functional Characterization of the Internal Ribosome Entry Site of eIF4G mRNA

“…The 5Ј-UTRs of BiP (7), Ultrabithorax (12), and Antennapedia (8) mRNAs are, respectively, 220, 968, and 252 nt, but the locations and sizes of the IRESs within them are not known. In the case of the 318-nt 5Ј-UTR of fibroblast growth factor 2 mRNA, the IRES resides in a 165-nt segment (9).…”

“…In the latter case, ribosomes are directed to internal AUGs by an internal ribosome entry site (IRES). 1 Internal initiation has been demonstrated by both in vitro and in vivo experimentation for picornaviruses (3), certain other viruses (4 -6), and a growing number of cellular mRNAs (7)(8)(9)(10)(11)(12).…”