Conformations of the single-stranded DNA of bacteriophage M13

At least two conformations of M13 single-stranded DNA have been demonstrated by measuring differences in sedimentation coefficient and by direct visualization in the electron microscope. Which form is obtained from infected cells and/or intact phage depends on the pH, ionic strength, and temperature. The slower-sedimenting form can be converted to the faster-sedimenting, single-stranded form by low ionic strength, alkali treatment, formamide, or formaldehyde, but not by exposure to 100 degrees C in 1.0 M NaCl. The ability to assume either conformation appears to be a function of the nucleic acid alone. Whether or not these different conformations are of biological significance is still unknown.