RNA polymerase II subunit B1 C-terminal domain (CTD) phosphatase that dephosphorylates 'Ser-5' of the CTD and regulates RNA polymerase II during the transition from 'Ser-5' to 'Ser-2' phosphorylation. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Interacts with RPO21. Shuttles constitutively between the cytoplasm and the nucleus. Present with 5480 molecules/cell in log phase SD medium. Belongs to the RPAP2 family.