THE 54-KDA RNA-BINDING PROTEIN FROM MUSTARD CHLOROPLASTS MEDIATES ENDONUCLEOLYTIC TRANSCRIPT-3' END FORMATION INVITRO

A 54 kDa protein from mustard chloroplasts was previously shown to int eract specifically with a conserved U-rich sequence element in RNA der ived from the 3' flanking regions of the plastid trnK and rps16 genes, which code for tRNA(Lys) and ribosomal protein CS19, respectively (Ni ckelsen and Link, 1991). This RNA-binding protein has now been purifie d by affinity chromatography on heparin Sepharose and poly(U) Sepharos e. In vitro processing experiments and nuclease Sl analyses of the pro cessing products revealed that the 54 kDa polypeptide is an endonuclea se. The in vitro cleavage sites are consistent with the positions of c orresponding transcript in vivo 3' ends downstream of trnK and rps16, suggesting that RNA 3' end formation takes place endonucleolytically a lso in vivo.