DNA elongation rates and growing point distributions of wild-type phage T4 and a DNA-delay amber mutant

The rates of DNA elongation by wild-type phage T4 and a gene 52 DNA-delay am mutant were estimated by pulse-labeling infected cells with tritiated thymidine and visualizing the gently extracted DNA by autoradiography. The estimated rate of chain elongation of wild-type DNA was 749 nucleotides/second early in synthesis and 516 to 581 nucleotides/second at a later time. The rate of DNA elongation by the am mutant was measured to be 693, 758 and 829 nucleotides/second during successive stages of synthesis, indicating that elongation was not slower than in wild-type. The kinetics of DNA increase after infection of host cells by wild-type phage T4 or by the gene 52 DNA-delay am mutant was followed using [methyl-³H]thymidine uptake into acid-insoluble material. It was found that DNA increase in both wild-type and am infections could be represented as exponential during early times and linear during late times of DNA synthesis. From the rates of DNA increase and the rates of DNA elongation we were able to estimate the number of growing points per chromosome equivalent of template DNA during the exponential and linear phases. Our estimates for wild-type phage were 0.55 and 0.71 to 0.80 growing points per chromosome equivalent of template DNA in the exponential and linear phases, respectively. For the am mutant we found 0.14 and 0.12 to 0.13 growing points per chromosome equivalent of template DNA during the exponential and linear phases, respectively. The apparent lower incidence of growing points in the am mutant infections suggests that the mutant may be defective in the initiation of growing points.