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 chloroplast psbA mRNA correlates with light-regulated translational activation of this message. We have isolated a cDNA encoding the main psbA RNA binding protein, RB47, and identified this protein as a member of the poly(A) binding protein family. Poly(A) binding proteins are a family of eukaryotic, cytoplasmic proteins thought to bind poly(A) tails of mRNAs 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 mature 47-kDa protein. RB47 expressed and purified from Escherichia coli binds to the psbA 5' UTR with similar specificity and affinity as RB47 isolated from C. reinhardtii chloroplasts. The identification of a normally cytoplasmic translation factor in the chloroplast suggests that the prokaryotic-like chloroplast translation machinery utilizes a eukaryotic-like initiation factor to regulate the translation of a key chloroplast mRNA. These data also suggest that poly(A) binding proteins may play a wider role in translation regulation than previously appreciated.