Chromatin-associated microprocessor assembly is regulated by PRP40, the U1 snRNP auxiliary protein

Cotranscriptional processing of RNA polymerase II-generated primary transcripts is a well-documented phenomenon. We recently showed that in plants, miRNA biogenesis is also a cotranscriptional event. Here, we report that Arabidopsis PRP40, the U1 snRNP auxiliary protein, positively regulates the recruitment of SE, the core component of the plant microprocessor, to miRNA genes. The association of DCL1, the microprocessor endoribonuclease, with chromatin was altered in prp40ab mutant plants. Impaired cotranscriptional microprocessor assembly was accompanied by RNA polymerase II accumulation at miRNA genes and retention of miRNA precursors at their transcription sites in the prp40ab mutant plants. We show that cotranscriptional microprocessor assembly, regulated by AtPRP40, positively affects RNAPII transcription of miRNA genes and is important to reach the correct levels of produced miRNAs.