Arabidopsis plants contain a CK2-like activity that phosphorylates CCA1 in vitro. GST (lanes 1 and 4) or GST-CCA1 (lanes 2, 3, 5–7) were incubated with 160 μg of WCE in the absence (lanes 1, 2, 4, and 5) or presence (lane 3) of 10 mM 2,3-diphosphoglycerate (D.G.), with recombinant CK2 (rCK2) in the absence (lane 6) or presence of 10 mM 2,3-diphosphoglycerate (lane 7) together with [γ-32P]ATP (lanes 1–3, 6, and 7) or [γ-32P]GTP (lanes 4 and 5). The arrow indicates the position of the full length GST-CCA1 protein.