STRUCTURE OF DNA OF BACTERIOPHAGE FD .2. ISOLATION AND CHARACTERIZATION OF A DNA FRACTION WITH DOUBLE STRAND-LIKE PROPERTIES

By enzymic degradation with two single strand-specific nucleases, exonuclease I from Escherichia coli and endonuclease from Neurospora crassa, a double strand-like DNA core has been detected in the single-stranded DNA of bacteriophage fd. From the kinetics of exonuclease I action at 30 °C the resistant DNA core has been estimated to represent 2.5% of the total DNA, corresponding to 150 out of 6000 nucleotide residues. In preparative experiments about 1.5% of the total nucleotides in fd DNA were isolated after limited nuclease digestion as core DNA fragments with an average chain length of about 40 nucleotide residues. In addition to its resistance to degradation by single strand-specific nucleases, fd DNA core is characterized by a high G + C-content, a simple pyrimidine distribution, and double strand-like spectral properties. Fd DNA core chains rapidly recover their secondary structure following denaturation. The data are interpreted in terms of a secondary structure of a single polynucleotide chain folding back on itself. © 1969.