MAC3A and MAC3B, Two Core Subunits of the MOS4-Associated Complex, Positively Influence miRNA Biogenesis.

MAC3A and MAC3B are conserved U-box-containing proteins in eukaryotes. They are subunits of the MOS4-associated complex (MAC) that plays essential roles in plant immunity and development in However, their functional mechanisms remain elusive. Here, we show that Arabidopsis MAC3A and MAC3B act redundantly in microRNA (miRNA) biogenesis. Lack of both MAC3A and MAC3B in the double mutant reduces the accumulation of miRNAs, causing elevated transcript levels of miRNA targets. also decreases the levels of primary miRNA transcripts (pri-miRNAs). However, MAC3A and MAC3B do not affect the promoter activity of genes encoding miRNAs ( genes), suggesting that they may not affect transcription. This result, together with the fact that MAC3A associates with pri-miRNAs in vivo, indicates that MAC3A and MAC3B may stabilize pri-miRNAs. Furthermore, we find that MAC3A and MAC3B interact with the DCL1 complex that catalyzes miRNA maturation, promote DCL1 activity, and are required for the localization of HYL1, a component of the DCL1 complex. Besides MAC3A and MAC3B, two other MAC subunits, CDC5 and PRL1, also function in miRNA biogenesis. Based on these results, we propose that MAC functions as a complex to control miRNA levels through modulating pri-miRNA transcription, processing, and stability.