Hydrodynamic properties and structure of fd virus

Abstract

A length of 8950 ± 200 Å and a diameter of 90 ± 10 Å have been obtained for fd virus from a simultaneous solution of the Broersma equations relating the length and diameter of a rod-like particle to its rotational, D R, and translational, D T, diffusion coefficients. Measurements of D R were by transient electric birefringence, and of D T by low-angle intensity fluctuation spectroscopy. A mass of (16.4 ± 0.6) × 10 6 daltons was calculated from the Svedberg equation using our measured values of D T, the sedimentation coefficient and the density increment. These results, together with the molecular weight of fd DNA, give a total number of major coat protein subunits of 2710 ± 110 and a ratio of nucleotides to protein subunits which is definitely non-integral, 2.30 ± 0.11. These measurements help delineate significant structural differences between fd and other filamentous viruses. Also included in this paper is an Appendix (by L. A. Day & S. A. Berkowitz) concerning the number of nucleotides, 6370 ± 140, and the density and refractive index increments of fd DNA.