Based on these data, we propose that dephosphorylation of U2 and U5 snRNP components by PP1/PP2A family phosphatases facilitates essential structural rearrangements in the spliceosome during the transition from the first to the second step..     ///    PP1/PP2A phosphatases are required for the second step of Pre-mRNA splicing and target specific snRNP proteins.. 