MULTIPLE ELEMENTS IN THE 5'-UNTRANSLATED REGION DOWN-REGULATE C-SIS MESSENGER-RNA TRANSLATION

Expression of the platelet derived growth factor (PDCF) and-chain, the product of the c-sis pi oto-oncogene, is regulated both at the transcriptional and translational level. Previous studies have shown: that the long 5' untranslated region (UTR) of the c-sis mRNA strongly inhibits synthesis of the PDGF-B chain. However, the assignments of down-regulatory regions within the 5' UTR were ambiguous. Expression of several site-directed point and deletion mutants of the 5' UTR of the c-sis mRNA in COS1 cells revealed that the UTR inhibited PDGF-B chain synthesis in a more complex manner than indicated by the previous studies. Abrogation of the three upstream short open reading frames by mutating each of the AUGs did not have any effect on the synthesis of the PDGF-B chain. Expression of deletion mutants revealed two partially overlapping regions, nucleotides 1-651 and 475-1022, each of which independently inhibited c-sis mRNA translation as effectively as the entire 5' UTR. Each of these regions contains a potentially strong stem-loop structure and a CC-rich element. These elements of the alternate down-regulatory regions could interact within the same region and/or with the elements of the other regulatory region to block c-sis mRNA translation. We show, in contrast to the previous reports, that the inhibition of c-sis mRNA translation cannot be attributed exclusively to any particular predicted secondary structure or a CC-rich element within the 5' UTR. Instead, we demonstrate that the PDGF-B chain synthesis is stringently controlled by multiple structures and CC-rich elements within the 5' UTR, which appear to be evolutionarily conserved. Interestingly, sequences of the 5' UTR are more highly evolutionarily conserved than those of the coding region.