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 CPL1, required for male gametes fertility. Multifunctional regulator that modulates plant growth, stress, and phytohormones responses. Positive transcription regulator of genes involved in high salinity resistance and auxin mediated signaling pathway. 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 RCF3. A number of isoforms are produced. According to EST sequences. Expressed in seedlings, particularly in tissues undergoing cell division and expansion, petioles, cotyledons vasculature, leaves, shoot and root meristems, and flowers, especially in buds and organs vasculatures. Leaf expansion defects, early flowering, low fertility, and increased salt (NaCl) sensitivity. Produces shorter hypocotyls than wild-type plants in the light, but not in the dark.