Induction of prophage SPO2 in Bacillus subtilis: isolation of excised prophage DNA as a covently closed circle

Bacillus subtilis tryC2, thyA, thyB, lysogenic for the phage DNA polymerase negative mutant SPO2 susL244, was induced under conditions preventing phage and bacterial DNA synthesis. The biological activity of DNA from induced cells and from uninduced controls was assayed by transformation and transfection, respectively. About 50% of the phage DNA biological activity in DNA extracted from induced cells was resistant to exposure to pH 11.8 TO 11.9. This DNA was operationally defined as alkali-resistant phage DNA. Transforming bacterial DNA from uninduced or induced cells and transfecting DNA from uninduced cells were more than 95% inactivated after exposure to high pH. The alkali-resistant phage DNA was characterized by sucrose gradient centrifugation, by centrifugation in cesium chloride-propidium iodide, and by electron microscopy. It was found to consist of a majority of covalently closed circular DNA molecules. Length measurements of a few relaxed circular molecules indicate a molecular weight of these similar to that previously found for mature SPO2DNA. Attempts to isolate similar covalently closed circular phage DNA from induced bacteria lysogenic for SPO2 phage with a functional DNA polymerase gene were unsuccessful. The gene order in mature and prophage SPO2 was determined by rescue of single and double markers from the respective type of DNA. The data obtained show that prophage DNA is (genetically) permuted relative to mature DNA. The phage attachment site is suggested to be located between genes I and J.