KSRP promotes the maturation of a group of miRNA precursors.

microRNAs (miRNAs) are small noncodingRNAs that down-regulate gene expression by reducing stability and/or translation of target mRNAs. In animals, miRNAs arise from sequential processing of hairpin primary transcripts by two RNAse III domain-containing enzymes, namely Drosha and Dicer, to generate a mature form of about 22 nucleotides. In this chapter we discuss our latest findings indicating that KSRP is an integral component of both Drosha and Dicer complexes. KSRP binds to the terminal loop sequence of a subset of miRNA precursors promoting their maturation. Our data indicate that the terminal loop is a pivotal structure where activators of miRNA processing as well as repressors of miRNA processing act in a coordinated way to convert cellular signals into changes in miRNA expression processing. This uncovers a new level of complexity of miRNA mechanisms for gene expression regulation.