The poly A binding protein RB47 was purified from C. reinhardtii chloroplasts and was found to have endoribonuclease activity. MS analysis suggested that an internal non-conserved linker (NCL)sequence was absent from the native protein. Recombinant, full length RB47 protein, including the NCL did not have endoribonuclease activity while versions lacking the NCL were active. An in vitro system, utilizing recombinant RB47 and chloroplast lysates, recreated the processing of RB47 in that the NCL was excised and the flanking sequences were spliced together.