Regulation of Unr expression by 5'- and 3'-untranslated regions of its mRNA through modulation of stability and IRES mediated translation.

Unr (upstream of N-ras) is a cytoplasmic RNA-binding protein that can act as a regulator of mRNA stability and IRES-mediated translation. Unr, a member of the cold-shock domain (CSD) protein super-family, is ubiquitously expressed, with variable abundance, in different tissues or during embryonic development. Prokaryotic and eukaryotic cold-shock protein expression is highly regulated at both the transcriptional and post-transcriptional levels. Here we analyzed the role of the 5'- and 3'-untranslated regions (UTR) of unr mRNA in post-transcriptional regulation of Unr expression. We show that, in vitro, unr 3'-UTR specifically destabilizes unr transcripts. Accordingly, in vivo, the half-life of unr messages deleted of noncoding regions is increased by approximately 3.6 fold, resulting in an enhanced steady-state level of Unr protein. We also show that the 5'-UTR exhibits IRES activity both when translated in vitro and in transiently transfected cells. This IRES activity displays cell type specificity with a higher efficiency in HeLa and HuH7 than in ES cells. Moreover, Unr IRES activity was higher in unr(-/-) than in unr(+/+) ES cells, indicating that Unr negatively regulates its own IRES activity. Our studies further reveal that Unr specifically interacts with its own mRNAs in vivo. These results suggest that a feedback control of mRNA translation is involved in regulating Unr expression.

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