The Protein Phosphatase 4 Complex Promotes Transcription and Processing of Primary microRNAs in Arabidopsis

Protein phosphatase 4 (PP4) is a highly conserved serine/threonine protein phosphatase in yeast, plants, and animals. The composition and functions of the plant PP4 are poorly understood. Here, we uncovered previously unsuspected complexity of Arabidopsis PP4 and identified the composition of one form of PP4 containing the regulatory subunit PP4R3A. We showed that PP4R3A, together with one of two redundant catalytic subunit genes PPX1 and PPX2, promotes the biogenesis of microRNAs. PP4R3A is a chromatin-associated protein, interacts with RNA polymerase II (Pol II), and recruits Pol II to the promoters of MIR genes to promote their transcription. PP4R3A probably also promotes the cotranscriptional processing of miRNA precursors as it recruits the microprocessor component HYPONASTIC LEAVES1 (HYL1) to MIR genes and to nuclear dicing bodies. Moreover, we showed that hundreds of introns exhibit splicing defects in pp4r3a mutants. Together, this study revealed a role of Arabidopsis PP4 in transcription and nuclear RNA metabolism.