LAUSR

An important proportion of microRNA (miRNA) genes tend to lie close to each other within animal genomes. Such genomic organization is generally referred to as miRNA clusters. Even though many miRNA clusters have been greatly studied, most attention has been usually focused on functional impacts of clustered miRNA co‐expression. However, there is also another compelling aspect about these miRNA clusters, their polycistronic nature. Being transcribed on a single RNA precursor, polycistronic miRNAs benefit from common transcriptional regulation allowing their coordinated expression. And yet, numerous reports have revealed striking discrepancies in the accumulation of mature miRNAs produced from the same cluster. Indeed, the larger polycistronic transcripts can act as platforms providing unforeseen post‐transcriptional regulatory mechanisms controlling individual miRNA processing, thus leading to differential miRNA expression, and sometimes even challenging the general assumption that polycistronic miRNAs are co‐expressed. In this review, we aim to address the current knowledge about how miRNA polycistrons are post‐transcriptionally regulated. In particular, we will focus on the mechanisms occurring at the level of the primary transcript, which are highly relevant for individual miRNA processing and as such have a direct repercussion on miRNA function within the cell.