A POLY(A) BINDING-PROTEIN FUNCTIONS IN THE CHLOROPLAST AS A MESSAGE-SPECIFIC TRANSLATION FACTOR

High-affinity binding of a set of proteins with specificity for the 5' untranslated region (UTR) of the Chlamydomonas reinhardtii chloroplas t psbA mRNA correlates with light-regulated translational activation o f this message. We have isolated a cDNA encoding the main psbA RNA bin ding protein, RB47. and identified this protein as a member of the pol y(A) binding protein family, Poly(A) binding proteins are a family of eukaryotic, cytoplasmic proteins thought to bind poly(A) tails of mRNA s and play a role in translational regulation. In vitro translation of RNA transcribed from the RB47 cDNA produces a precursor protein that is efficiently transported into the chloroplast and processed to the m ature 47-kDa protein. RB47 expressed and purified from Escherichia col i binds to the psbA5' UTR with similar specificity and affinity as RB4 7 isolated from C. reinhardtii chloroplasts. The identification of a n ormally cytoplasmic translation factor in the chloroplast suggests tha t the prokaryotic-like chloroplast translation machinery utilizes a eu karyotic-like initiation factor to regulate the translation of a key c hloroplast mRNA, These data also suggest that poly(A) binding proteins may play a wider role in translation regulation than previously appre ciated.