Mechanism of DNA Chain Growth, IV. Direction of Synthesis of T4 Short DNA Chains as Revealed by Exonucleolytic Degradation

T4 nascent short chains labeled at their growing ends with H3-thymidine and uniformly with C14-thymidine were prepared, separated into complementary strands, and degraded by E. coli exonuclease I in the 3′to 5′direction or by B. subtilis nuclease in the 5′to 3′direction. The kinetics of release of H3and C14labels by both enzymes was consistent with the conclusion that the H3label is at the 3′end of the nascent short chains of both strands and that the short chains are products of discontinuous synthesis in the 5′to 3′direction along the two template strands.