ERH promotes primary microRNA processing beyond cluster assistance.

MicroRNA (miRNA) maturation is initiated by the Microprocessor complex, comprising DROSHA and DGCR8, that processes primary miRNAs (pri-miRNAs). Recent studies have identified ERH and SAFB2 as auxiliary factors that enhance the functionality of the Microprocessor. These factors are required for cluster assistance, where optimal pri-miRNAs facilitate the processing of adjacent suboptimal pri-miRNAs. However, the specific action mechanisms of ERH and SAFB2 have not yet been defined. In this study, we found that ERH broadly enhances the processing of pri-miRNAs regardless of their genomic contexts, affecting both stand-alone and clustered ones. Suboptimal hairpins are affected more prominently by ERH knockdown than efficiently processed hairpins. In contrast, SAFB2 specifically supports the processing of suboptimal pri-miRNA hairpins within clusters. This study reveals the distinct roles of ERH and SAFB2 in cluster assistance and presents a new model, in which SAFB2 facilitates the Microprocessor's transfer between hairpins, while ERH enables the efficient processing of suboptimal pri-miRNAs.