Effect of 3'UTR length on the translational regulation of 5'-terminal oligopyrimidine mRNAs.

In Vertebrates, all genes coding for ribosomal proteins, as well as those for other proteins implicated in the production and function of translation machinery, are regulated by mitogenic and nutritional stimuli, at the translational level. A cis-regulatory element necessary for this regulation is the typical 5VUTR, common to all ribosomal protein mRNAs, which always starts at the 5V end with several pyrimidines. Having noticed that the 3VUTR of all ribosomal protein mRNAs is much shorter than most cellular mRNAs, we have now studied the possible implication of this 3VUTR feature in the translational regulation. For this purpose, we constructed a number of chimeric genes whose transcribed mRNAs contain: (1) the 5VUTR of ribosomal protein S6 mRNA or, as a control, of h-actin mRNA; (2) the EGFP reporter coding sequence from the starting AUG to the stop codon; (3) different 3VUTRs of various lengths. These constructs have been stably transfected in human HEK293 cells, and the translation regulation of the expressed chimeric mRNAs has been analyzed for translation efficiency, in growing and in serum starved cells, by the polysome association assay. The results obtained indicate that, while the typical growth-associated translational regulation is bestowed on an mRNA by the pyrimidine sequence containing 5VUTR, the stringency of regulation depends on the short size of the 3VUTR.