Sequence and translation initiation properties of the Xenopus TGFβ5, PDGF-A, and PDGF-α receptor 5′ untranslated regions

The properties of the architecturally complex Xenopus laevis TGFβ5, PDGF-A and PDGF-α receptor 5′UTRs were investigated. 5′ extended cDNAs were obtained by 5′RACE, resulting in long 5′UTRs (478-710nt) with multiple upstream AUGs (3-13), and the potential to fold into stable structures. Injection studies suggested that the cloned PDGF-αR5′UTR contains an intron. Splicing at potential 5′ and 3′ splice sites would result in a non-complex 5′UTR of 142 nt. The above mentioned 5′UTR characteristics are inhibitory for ribosomal scanning. Indeed, relative to the βglobin 5′UTR, the complex 5′UTRs strongly repressed initiation of protein synthesis in pre-MBT Xenopus embryos. However, later in embryogenesis, the inhibition was partly relieved. The results show temporal translational control by these 5′UTRs. Transgenic embryos showed that the 5′UTRs allowed translation throughout the embryo; spatial control could not be observed. Interestingly, a fragment in the PDGF-A 5′UTR highly similar to an element in the human PDGF-A 5′UTR is complementary to Xenopus 18S ribosomal RNA. None of these Xenopus 5′UTRs contains an IRES, as determined by injecting bicistronic constructs.