The 54 kDa RNA-binding protein from mustard chloroplasts mediates endonucleolytic transcript 3' end formation in vitro.

A 54 kDa protein from mustard chloroplasts was previously shown to interact specifically with a conserved U-rich sequence element in RNA derived from the 3' flanking regions of the plastid trnK and rps16 genes, which code for tRNA(Lys) and ribosomal protein CS19, respectively (Nickelsen and Link, 1991). This RNA-binding protein has now been purified by affinity chromatography on heparin Sepharose and poly(U) Sepharose. In vitro processing experiments and nuclease S1 analyses of the processing products revealed that the 54 kDa polypeptide is an endonuclease. The in vitro cleavage sites are consistent with the positions of corresponding transcript in vivo 3' ends downstream of trnK and rps16, suggesting that RNA 3' end formation takes place endonucleolytically also in vivo.