Posttranscriptional gene regulation relies on hundreds of RNA binding proteins (RBPs) but the function of most RBPs is unknown. The human RBP HuR/ELAVL1 is a conserved mRNA stability regulator. We used PAR-CLIP, a recently developed method based on RNA-protein crosslinking, to identify transcriptome-wide ∼26,000 HuR binding sites. These sites were on average highly conserved, enriched for HuR binding motifs and mainly located in 3' untranslated regions. Surprisingly, many sites were intronic, implicating HuR in mRNA processing. Upon HuR knockdown, mRNA levels and protein synthesis of thousands of target genes were downregulated, validating functionality. HuR and miRNA binding sites tended to reside nearby but generally did not overlap. Additionally, HuR knockdown triggered strong and specific upregulation of miR-7. In summary, we identified thousands of direct and functional HuR targets, found a human miRNA controlled by HuR, and propose a role for HuR in splicing.
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