PROPERTIES OF A NOVEL OLIGONUCLEOTIDE-RELEASING BIDIRECTIONAL DNA EXONUCLEASE FROM MOUSE MYELOMA

Highly purified, but not homogeneous, samples of helix-destabilizing protein 1 from mouse myeloma contain a novel oligonucleotide-releasing DNA exonuclease. This enzyme was separated from helix-destabilizing protein 1 and obtained in highly purified form. A polypeptide of MW 41,000 is a main constituent of the purified enzyme, and this polypeptide comigrated with the exonuclease activity during the final step of the purification, Sephacryl S-200 gel filtration where the enzyme had a native MW of 40,000. Overall purification of enzyme activity was > 20,000-fold. This exonuclease releases 5''-oligonucleotides in a limited processive manner in both the 5'' .fwdarw. 3'' and 3'' .fwdarw. 5'' directions. Activity of the enzyme is resistant to 1 mM N-ethylmaleimide, requires a divalent cation, has an alkaline pH optimum and degrades single-stranded DNA much faster than double-stranded DNA or RNA. The predominant oligonucleotide product with uniformly labeled substrates is (pdN)2. With 3'' end labeled substrates, > 95% of the labeled products are (pdN)4 and (pdN)5; with 5'' end labeled substrates, the main labeled product is (pdA)2. The rate of product release from 3'' and 5'' end labeled substrates is nearly identical at 37.degree. C. A model of the action of this enzyme and a comparison with a human placenta exonuclease are discussed.