DNA and protein lattice-lattice interactions in the filamentous bacteriophages

The relations between the protein coats and DNAs of the four filamentous bacteriophages fd, Xf, Pf1, and Pf3 are considered. These viruses have similar morphologies, yet show a diversity of detailed structure, having different protein coat symmetries (helical and rotational), different coat protein sizes (44-50 amino acids per subunit) and sequences, different nucleotide axial translations (2.3-5.5 A), and different ratios of nucleotides per coat protein subunit (integers 1.0 and 2.0, and nonintegers approximately 2.4). These divergences are all reconciled quantitatively by means of two theoretical concepts: the pitch connection and the restricted pitch connection. The pitch connection relates protein and DNA surface lattices with arbitrary, nonintegral nucleotide/subunit ratios in a nonrandom way. The restricted pitch connection selects a preferred set of n/s values. Both relations are derived formally in a mathematical appendix. The available structural data are explained, including the fd DNA pitch indicated by x-ray diffraction photos and the similar DNA morphologies of Xf and fd. Predictions are made for the existence of nonclassical inverted DNA structures (I-DNA) in Pf1 and Pf3.