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The chemical and physical properties of deoxyribonucleic acid (DNA) from transducing phage PBS 1 differ considerably from those of host Bacillus subtilis DNA. The guanine plus cytosine (G + C) content of PBS 1 DNA is 28%, and thymidylic acid is replaced completely by deoxyuridylic acid. The buoyant density (1.722 g/cc) is unusually high, and the Tm (76.5 C) is extremely low for its G + C content (I. Takahashi and J. Marmur, Nature 197:784, 1963). In spite of these unusual properties, PBS 1 is capable of transducing a number of genetic markers in B. subtilis (I. Takahashi, Biochem. Biophys. Res. Commun. 5:171, 1961). Furthermore, it has been shown that the frequencies of joint transfer of genetic markers in transduction mediated by PBS 1 are always higher than those observed in transformation. It is therefore of interest to investigate the molecular size of PBS 1 DNA. We report here that the molecular weight of DNA extracted from active (plaque-forming) PBS 1 particles is about 1.5 times greater than that of T2 DNA, which is the largest phage DNA hitherto reported. Properties of transducing particles and transducing fragments will be reported elsewhere. As PBS 1 DNA contains uracil in place of thymine, the DNA was labeled with radioactive uridine. To obtain labeled PBS 1 particles, B. subtilis strain A26 (uracil-) was infected in Difco Nutrient Broth containing 5 ,ug of uracil per ml and 20,c of uridine-6-3H per ml (9.3 c/mmole). The phage was propagated and purified as described previously (I. Takahashi, J. Gen. Microbiol. 31:211, 1963; I. Takahashi and J. Marmur, Biochem. Biophys. Res. Commun. 10:289, 1963). To separate active particles from transducing particles, the purified phage suspension was centrifuged in a CsCl solution having a mean density of 1.422 g/cc for 20 hr at 30,000 rev/min at 20 C in a Spinco SW50 rotor. The plaque-forming particles banded at a position corresponding to a density of 1.426 g/cc, which is about 0.010 g/cc denser than that of transducing particles. The difference in density between the two types of particles may be primarily due to the fact that the density of PBS 1 DNA is 0.019 g/cc greater than that of B. subtilis DNA. The peak fraction for plaque-forming activity was used for the extraction of DNA. 32P-labeled T2 particles were prepared and purified by a method similar to that described by H. Yamagishi, F. Yoshizako, and K. Sato (Virology 30:29, 1966). The method described by the above authors was used to extract DNA from both PBS 1 and T2. Zone centrfugation in sucrose gradients was carried out, and the molecular weight was calculated by

Molecular weight of bacteriophage PBS 1 deoxyribonucleic acid