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Normal preparations of B. subtilis DNA have weight average native molecular weights of 10 to 30 x 10(6). For any given preparation the upper and lower 95% size limits may differ by a factor of ten or more. Single-stranded molecular weights indicate an average of 1 to 4 breaks per single strand of the native DNA. The reduction in transforming activity and viscosity following DNAase I digestion can be accounted for by a direct relationship between the transforming activity of a DNA and its single-stranded molecular weight. Uptake studies with DNAase I treated heavy ((2)H(15)N (3)H) DNA show that single strand breaks inhibit integration less than transformation. A provisional estimate of the size of the integrated region based on correlating the single strand size of the donor-recipient complex with the donor-recipient density differences following alkali denaturation came to 1530 nucleotides. Using a competent, nonleaky thymine-requiring strain of B. subtilis grown in 5-BU medium before and after transformation, it was shown that (a) No detectable amount of DNA synthesis is necessary for the initial stages of integration, (b) Cells which have recently been replicating DNA are not competent. (c) Cells containing donor DNA show a lag in DNA replication following transformation, (d) When donor DNA is replicated it initially appears in a density region between light and hybrid. This indicates that it includes the transition point formed at the time of reinitiation of DNA synthesis in the presence of 5-BU following transformation. A model is proposed in which donor DNA is integrated at the stationary growing point of the competent cell, which is in a state of suspended DNA synthesis.
Walter F. Bodmer (Fri,) studied this question.