The PROTEIN PHOSPHATASE4 Complex Promotes Transcription and Processing of Primary microRNAs in Arabidopsis.

PROTEIN PHOSPHATASE4 (PP4) is a highly conserved Ser/Thr protein phosphatase found in yeast, plants, and animals. The composition and functions of PP4 in plants are poorly understood. Here, we uncovered the complexity of PP4 composition and function in Arabidopsis () and identified the composition of one form of PP4 containing the regulatory subunit PP4R3A. We show that , together with one of two redundant catalytic subunit genes, () and , promotes the biogenesis of microRNAs (miRNAs). PP4R3A is a chromatin-associated protein that interacts with RNA polymerase II and recruits it to the promoters of miRNA-encoding () genes to promote their transcription. PP4R3A likely also promotes the cotranscriptional processing of miRNA precursors, because it recruits the microprocessor component HYPONASTIC LEAVES1 to genes and to nuclear dicing bodies. Finally, we show that hundreds of introns exhibit splicing defects in mutants. Together, this study reveals roles for Arabidopsis PP4 in transcription and nuclear RNA metabolism.