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 5′UTR, common to all ribosomal protein mRNAs, which always starts at the 5′ end with several pyrimidines. Having noticed that the 3′UTR of all ribosomal protein mRNAs is much shorter than most cellular mRNAs, we have now studied the possible implication of this 3′UTR feature in the translational regulation. For this purpose, we constructed a number of chimeric genes whose transcribed mRNAs contain: (1) the 5′UTR of ribosomal protein S6 mRNA or, as a control, of β-actin mRNA; (2) the EGFP reporter coding sequence from the starting AUG to the stop codon; (3) different 3′UTRs 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 5′UTR, the stringency of regulation depends on the short size of the 3′UTR.