A 5' EXORIBONUCLEASE FROM CYTOPLASMIC EXTRACTS OF MOUSE SARCOMA-180 ASCITES-CELLS

An exonuclease that appears to represent the predominant nuclease activity in cytoplasmic extracts of sarcoma 180 ascites cells has been partially purified and characterized. The enzyme attacks RNA chains in a 5' to 3' direction, and releases 5'-mononucleotides. The initial cleavage, however, can occur at either the first, second and probably third phosphodiester linkage in some RNAs. The enzyme attacks transcripts terminated with a 5'-triphosphate more slowly than those with a 5' monophosphate, and releases a compound larger than GTP from transcripts that begin with a pppG. Capped transcripts are cleaved at least as readily as those with a 5'-P, yielding a compound larger than 7mGpppGm. The occurrence of an such an exonuclease capable of attacking capped RNAs would make it possible for mammalian cells to initiate mRNA degradation by a 5' exonucleolytic mechanism.