Positive Regulation of ABA Responses by AtCPL5 that Encodes a Novel Ser-2-specific RNA Polymerase II C-terminal Domain Phosphatase in Arabidopsis

Arabidopsis RNA polymerase II (RNAPII) C-terminal domain (CTD) phosphatases regulate stress-responsive gene expression and plant development via the dephosphorylation of serine residues of the CTD. Some of these phosphatases (CPL1-CPL3) negatively regulate ABA and stress responses. Here, we isolated AtCPL5, a cDNA encoding two CTD phosphatase domain (CPD)-containing protein. To characterize AtCPL5, we analyzed the gene expression patterns and subcellular protein localization, investigated various phenotypes of AtCPL5-overexpressors and knockout mutants involved in ABA and drought responses, performed microarray and RNA hybridization analyses using AtCPL5-overexpressors, and assessed CTD phosphatase activities of the purified AtCPL5 and each CPD of the protein. Transcripts of the nucleus-localized AtCPL5 were induced by ABA and drought. AtCPL5-overexpressors exhibited ABA-hypersensitive phenotypes (increased inhibition of seed germination, seedling growth, and stomatal aperture), lower transpiration rates upon dehydration, and enhanced drought tolerance, while the knockout mutants showed weak ABA hyposensitivity. AtCPL5 overexpression changed expression of numerous genes, including the ones involved in ABA-mediated responses. In contrast to Ser-5-specific phosphatase activity of the negative stress response regulators, purified AtCPL5 and each CPD of the protein specifically dephosphorylated Ser-2 in RNAPII CTD. We conclude that AtCPL5 is a unique CPL family protein that positively regulates ABA-mediated development and drought responses in Arabidopsis.