The identification of an endonuclease that cleaves within an HuR binding site in mRNA

Messenger RNAs (mRNAs) that contain U-rich elements are targeted for rapid decay. Selective inhibition of this decay results in a rapid increase in st eady state level. Thus, this is an important regulatory step in gene expres sion. Previously, we have found that these mRNAs are selectively stabilized by a specific mRNA binding protein called HuR. The mechanism of action of HuR is not well understood. It has been postulated that HuR stabilizes mRNA by the displacement or inhibition of factors that specifically cleave or d eadenylate these mRNAs. In this paper, we report the identification and cha racterization of a novel endonuclease that cleaves within an HuR binding si te in p27kip1 mRNA. The specificity of this endonuclease and its inhibition by HuR argue for it playing a role in the postranscriptional regulation of gene expression.