A Hexanucleotide Element Directs MicroRNA Nuclear Import

MicroRNAs (miRNAs) negatively regulate partially complementary target messenger RNAs. Target selection in animals is dictated primarily by sequences at the miRNA 5′ end. We demonstrated that despite their small size, specific miRNAs contain additional sequence elements that control their posttranscriptional behavior, including their subcellular localization. We showed that human miR-29b, in contrast to other studied animal miRNAs, is predominantly localized to the nucleus. The distinctive hexanucleotide terminal motif of miR-29b acts as a transferable nuclear localization element that directs nuclear enrichment of miRNAs or small interfering RNAs to which it is attached. Our results indicate that miRNAs sharing common 5′ sequences, considered to be largely redundant, might have distinct functions because of the influence of cis-acting regulatory motifs.