Expression of Calcium-Dependent Protein Kinase Gene (PnCDPK1) is Affected by Various Light Conditions in Pharbitis nil Seedlings

Calcium-dependent protein kinases (CDPKs) are central calcium signal decoders. Here we report the isolation and characterization of the Japanese Morning Glory's (Pharbitis nil) CDPK gene, termed PnCDPK1. The full-length cDNA of 1943 bp contains an open reading frame for PnCDPK1 consisting of 514 amino acid residues and a calculated molecular mass of 57.9 kDa. The deduced amino acid sequence suggests that this protein contains the kinase domain at the amino terminus and autoregulatory and calmodulin-like domain at the carboxy terminus. Sequence alignment indicated that PnCDPK1 shared high similarities with other CDPKs. Biochemical analyses showed that bacterially expressed recombinant protein was catalytically active and was able to phosphorylate the histone III-S in a calcium-dependent manner. Besides the identification of PnCDPK1 as a member of the CDPK family, it was shown that a transcript of PnCDPK1 was modulated during germination and seedling growth. Moreover, PnCDPK1 mRNA was present in every tested organ, including root, hypocotyl, and cotyledon of the light- and dark-grown plant; however, the higher expression level was found in dark-treated ones. The PnCDPK1 mRNA level in the cotyledons exhibited a significant increase after moving seedlings into the dark, peaking 2 and 8 h after dark exposure, and then it gradually decreased. Expression was significantly changed by exposure to red light after 8 h in the dark. These data unequivocally identify the product of the PnCDPK1 gene as a calcium-dependent protein kinase and emphasize the potential that this gene can be an element of light signal transduction involved in growth and development of P. nil.