Exonuclease VI, a new nuclease activity associated with E. coli DNA polymerase.

pH 7.9, and recentrifuged under identical conditions in another gradient of CsCl (average density p = 1.45) in the presence of an excess of actinomycin. The "hybrid" is more dense than d(A-T) in the first gradient, and less dense in the second. All "hybrids" were differentially labeled in the d(A-T) and d(DAP-T) segments, and purified in two successive gradients. Actinomycin was removed by phenol extraction following dialysis against Tris-HCl 0.01 M, pH 7.9. Abbreviations: j-mercaptoethanol; deoxynucleoside-5'-triphosphate; dTTP, dDAP-TP, deoxynucleoside_5'-triphosphate of adenine, thymine, and 2,6-diamino-purine, respectively; d(A-T), d(DAP-T), alternating deoxynucleotide copolymers of adenine and thymine, 2,6-diaminopurine and thymine, respectively; PCMB, p-chloromercuribenzoic acid; d(A-T)/d(DAP-T), density "hybrids" with pure d(A-T) segments at the 5' and pure d(DAP-T) at the 3' regions; cellulose;