Single-stranded DNA from the Kilham Rat Virus

Summary The nucleic acid of the Kilham rat virus was found to be single-stranded DNA. This was demonstrated by the lack of a distinct melting curve of the viral DNA, lack of reaction of actinomycin D with viral DNA, and the reaction of both intact virus and viral DNA with formaldehyde. The hydroxymethyl groups, added by the formaldehyde, did not interfere with complete recovery of the secondary structure of the extracted DNA. Electron micrographs of the DNA also showed pooling and sharp folds and turns which are characteristics of single-stranded DNA. When rat virus was centrifuged to equilibrium in caesium chloride density gradients, three peaks of haemagglutinating activity were found at densities of 1.43, 1.38 and 1.32 g./cm.3. The examination of material from these peaks showed the peak at 1.43 g./cm.3 contained full viral particles, the peak at 1.38 g./cm.3 contained a mixture of full and empty viral particles, and the peak at 1.32 g./cm.3 contained large amounts of debris in which an occasional viral particle was visible. It was concluded from these data that the intact viral particle had a density of 1.43 g./cm.3, and the protein coat had a density of 1.32 g./cm.3. The density of the DNA was found to be 1.72 g./cm.3; therefore, the intact viral particle contained 34% DNA. The average length of the DNA molecule extracted from the rat virus was found to be 1.30 µm., which gave a molecular weight of 1.2 × 106 for the DNA. With this value and the 34% value found for the DNA content of the viral particle, the molecular weight of the virus particle was estimated to be 3.6 × 106.