Processively dephosphorylates 'Ser-5' but not 'Ser-2' of the heptad repeats YSPTSPS in the C-terminal domain of the largest RNA polymerase II subunit (RPB1). This promotes the activity of RNA polymerase II. Together with CPL2, required for male gametes fertility. Multifunctional regulator that modulates plant growth, stress, and phytohormones responses. Negative regulator of stress gene transcription involved in abscisic acid (ABA) mediated and jasmonic acid (JA) mediated signaling pathways, NaCl, osmotic stress, wounding, and cold resistance. Regulates negatively the expression of jasmonic acid (JA) biosynthetic genes in response to wounding (PubMed:11874572, PubMed:12149434, PubMed:12149453, PubMed:15388846, PubMed:18506580, PubMed:18764923). Forms a complex with RCF3 that modulates co-transcriptional processes such as mRNA capping and polyadenylation, and functions to repress stress-inducible gene expression (PubMed:23874224). Dephosphorylates RCF3 (PubMed:26227967). Involved in the dephosphorylation of EIF4A3. This dephosphorylation retains EIF4A3 in the nucleus and limits its accumulation in the cytoplasm. Is essential for the degradation of the nonsense-mediated mRNA decay (NMD) transcripts (PubMed:26887918). H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds Mg(2+), Co(2+) or Mn(2+). Optimum pH is 5.5-6.0. Interacts with FREE1, ANAC019, MYB3, MYB4 and MYB32. Binds to DMS3 (PubMed:18541146). Interacts with RCF3 (PubMed:23874224, PubMed:24146632, PubMed:24303021, PubMed:26512101). Interacts with RS40 and RS41 (PubMed:24146632). Interacts with EIF4A3 (PubMed:26887918). Interacts with UPF3 (PubMed:26887918). Expressed at very low levels in roots, leaves, stems, flowers and siliques. Slightly repressed by ABA. DRBM domains are required for interactions with target transcription factors such as ANAC019 and MYB3. Grows more rapidly and flower later than wild-type plants. Increased tolerance to salt stress and to ABA during seed germination but more sensitive to freezing damage at the seedling stage, by the enhanced expression of abiotic stress-induced genes. Hypersensitivity to MeJA, accumulates high levels of anthocyanin on medium containing MeJA. Confers wound hyperresponsiveness of JA-biosynthetic genes.