Silver and mercury probing of deoxyribonucleic acid structures in the filamentous viruses fd, If1, IKe, Xf, Pf1, and Pf3. 
Ag+ binding and Hg2+ binding to both double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) have been examined in some detail, and the results have been applied to study the structures of circular ssDNA in several filamentous viruses.  It has been known for some time that Ag+ and Hg2+ bind to the bases of DNA producing characteristic large changes in absorbance and circular dichroism (CD) spectra, as well as changes in sedimentation rates.  In the case of Ag+, it is known that there are three modes of binding to isolated dsDNA, referred to as types I, II, and III.  Type III binding, by definition, occurs when Ag+ binds to Ag-dsDNA complexes having sites for binding types I and II extensively occupied, if not saturated.  It produces CD spectra, assigned in this study, and absorbance spectra that are isosbestic with those of the Ag-dsDNA complexes present prior to its onset.  In phosphate buffers binding is restricted to types I and II, whereas in borate buffers weaker type III binding can occur.  Characteristics of types I, II, and III were observed for the DNAs in fd, If1, IKe, and Xf, but not for those in Pf1 and Pf3.  Similarly, many of the spectral changes seen when Hg2+ binds to isolated double-stranded DNA are mimicked by Hg2+ binding to the DNAs within fd, IKe, If1, and Xf, but not for those in Pf1 and Pf3.  The Ag+ and Hg2+ results indicate the presence of right-handed DNA helices in fd, If1, IKe, and Xf, with the two antiparallel strands of the covalently closed single-stranded DNAs having the bases directed toward the virion axes.  For Pf1 and Pf3, Ag+ and Hg2+ binding cause large absorbance changes but only small CD changes.  The very different results for Pf1 and Pf3 are consistent with the presence of inverted DNA structures (I-DNA) with the bases directed away from the structure axes, but the two structures differ from one another.  Sedimentation velocity changes with Ag+ and Hg2+ binding strongly suggest structural linkages between the DNA and the surrounding protein sheath in each of the viruses.