Phylogenetic and functional features of the plastid transcription kinase cpCK2 from Arabidopsis signify a role of cysteinyl SH-groups in regulatory phosphorylation of plastid sigma factors.

A plastidic serine/threonine protein kinase, initially named plastid transcription kinase (PTK) has been implicated in phosphorylation and redox control of chloroplast transcription. This kinase was later renamed as chloroplast casein kinase 2 (cpCK2) because of its physical and functional similarity to nucleocytosolic casein kinase 2 (ncCK2). It shares all four of its cysteine residues with ncCK2 from land plants, while only three of these residues are conserved in algal CK2-type sequences, and just two in animals. Using bacterial overexpression of cpCK2 from Arabidopsis thaliana, here we show the principal features of this enzyme and assign functional determinants of its role as a transcriptional regulator in vitro. The recombinant protein is capable of using various plant sigma transcription factors as phosphorylation substrates. Electrophoretic mobility shift DNA-binding assays reveal differential effects of sigma phosphorylation, depending on the factor and the promoter used. Treatment of the kinase with redox-active reagents indicate a critical involvement of thiol groups in both its enzymatic activity and interaction capabilities. Mutational exchanges of cysteine to serine residues, in combination with in vitro assays, have provided clues to the possible role of individual cysteines. For instance, while Cys4 but not Cys2 is essential for activity, the latter seems to be involved in the formation of intermolecular (regulatory) disulfide bonds.