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 transc riptional and translational level. Previous studies have shown: that t he long 5' untranslated region (UTR) of the c-sis mRNA strongly inhibi ts synthesis of the PDGF-B chain. However, the assignments of down-reg ulatory regions within the 5' UTR were ambiguous. Expression of severa l 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 synthe sis in a more complex manner than indicated by the previous studies. A brogation 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 overlap ping regions, nucleotides 1-651 and 475-1022, each of which independen tly inhibited c-sis mRNA translation as effectively as the entire 5' U TR. Each of these regions contains a potentially strong stem-loop stru cture and a CC-rich element. These elements of the alternate down-regu latory regions could interact within the same region and/or with the e lements 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 partic ular predicted secondary structure or a CC-rich element within the 5' UTR. Instead, we demonstrate that the PDGF-B chain synthesis is string ently controlled by multiple structures and CC-rich elements within th e 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.