A 34 kDa ribonucleoprotein (p34) was purified to homogeneity from a 1.0 M KCl extract of spinach chloroplasts and characterized as an effective phosphate acceptor for casein kinase II (CK-II). The N-terminal 21 residues (W-V-A-Q-T-S-E-E-E-Q-E-G-S-T-N-A-V-L-E-G-E) of p34 were 95% identical with the sequence reported for 28RNP (plastid mRNA 3' end processing factor in chloroplast). Moreover, the findings that DNAs as well as RNAs significantly stimulate the CK-II catalyzed phosphorylation of p34 in vitro and induce its conformational change, suggest that the physiological activity of p34-bound RNA or DNA in chloroplast post-transcriptional regulation is controlled by specific p34 phosphorylation by CK-II.
