THE PHOTOSYSTEM-II SUBUNIT CP29 CAN BE PHOSPHORYLATED IN BOTH C3 AND C4 PLANTS AS SUGGESTED BY SEQUENCE-ANALYSIS

The CP29 subunit of Photosystem II is reversibly phosphorylated in Zen mays upon exposure to high light in the cold (Bergantino et al., J Bi ol Chem 270 (1995) 8474-8481). This phenomenon was previously proposed to be restricted to C4 plants. We present the complete sequence of th e CP29 protein, deduced from a maize Lhcb4 cDNA clone, and its compari son with the previously known Lhcb4 sequences of two C3 plants: Hordeu m vulgare and Arabidopsis thaliana. Despite the relatively low degree of homology in their amino-terminal region, i.e. the part of the molec ule which is phosphorylated in maize, the three polypeptides conserve consensus sequences for the site of phosphorylation. We proved by immu noblotting and P-33-labelling that the same post-translational modific ation occurs in barley. Being thus common to C3 and C4 plant species, the phosphorylation of this minor antenna complex of Photosystem II ap pears now as a widespread phenomenon, possibly part of the phosphoryla tion cascade which signals the redox status of the plastoquinone to th e nuclear transcription apparatus. Arabidopsis plants do not show phos phorylation of CP29 in the same conditions, but other low-molecular-we ight phosphoproteins, whose role need to be elucidated, become evident .