Identification of an RNA element that confers post-transcriptional repression of connective tissue growth factor/hypertrophic chondrocyte specific 24 (ctgf/hcs24) gene: Similarities to retroviral RNA-protein interactions

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Abstract
The repressive effect of the 3′-untranslated region (3′-UTR) in human connective tissue growth factor/hypertrophic chondrocyte specific 24 (ctgf/hcs24) mRNA on gene expression had been demonstrated in our previous study. Here, we identified a minimal RNA element in the 3′-UTR, which acts as a cis-acting element of structure-anchored repression (CAESAR). Deletion analyses of the 3′-UTR led us to minimize the element of 84 bases at the junction of the coding region and the 3′-UTR. The minimized RNA segment is predicted, and actually capable of forming a stable secondary structure in vitro. Mutational analyses disclosed a significant relationship between the predicted structure and repressive effect. The utility of CAESAR as a post-transcriptional regulatory element was represented by the fact that steady-state mRNA levels were not affected by CAESAR linked in cis, while protein levels from such a chimeric gene were markedly reduced. Of note, the CAESAR sequence exerted no effect, when it was placed upstream of the promoter. Finally, RNA gel electromobility-shift analyses demonstrated a nuclear factor that interacts with the folded CAESAR. Taken together, it was uncovered that CAESAR of ctgf is a novel post-transcriptional structured RNA regulatory element, probably acting through direct interactions with a nuclear factor as observed in retroviral RNA elements with certain proteins.