Diversifying microRNA sequence and function

Key Points MicroRNAs (miRNAs) are small non-coding RNAs that guide post-transcriptional gene regulation to shape the rate at which genetic information is converted into proteins. Due to this, miRNAs contribute to the establishment of gene expression patterns that are required for normal development and physiology in plants and animals. RNase III enzymes, together with specific double-stranded RNA-binding partner proteins, produce miRNAs from genomically encoded precursor miRNAs (pre-miRNAs). In rare cases, nucleases from other cellular pathways can replace RNase III enzymes in the production of miRNAs. After the assembly of miRNA duplexes, these small RNAs are loaded into proteins from the Argonaute (AGO) protein family. AGO proteins organize small RNAs into subdomains, including the seed sequence, which mediates target RNA binding. The mechanisms by which miRNAs function include endonucleolytic cleavage, translational repression and mRNA turnover. Recent evidence suggests that small RNA stability can be influenced by miRNA sequence motifs, chemical modifications and interactions with target mRNAs. miRNAs are annotated as single sequences, but recent high-throughput efforts to catalogue small RNAs from various organisms, tissues and cell types reveal that most miRNAs comprise multiple isoforms. Several mechanisms have been shown to diversify miRNA sequence and function. The advent of next-generation sequencing technology has revealed the miRNAs of key model organisms, but the extent to which each miRNA contributes to the regulation of targets in the transcriptome of a given cell type remains unclear. The biochemical and biophysical properties of miRNA silencing complexes provide a quantitative framework for their reciprocal function and their targets, according to their abundance and relative stoichiometry inside the cell. The mechanisms that regulate miRNA stability and the generation of distinct miRNA isoforms are beginning to be elucidated. Better understanding of how such miRNAs mediate gene expression control will require quantitative analyses that dissect different models of miRNA function. MicroRNAs (miRNAs) regulate the expression of most genes in animals, but we are only now beginning to understand how they are generated, assembled into functional complexes and destroyed. Various mechanisms have now been identified that regulate miRNA stability and that diversify miRNA sequences to create distinct isoforms. The production of different isoforms of