Structure of the Filamentous Phage Virion

Abstract

The filamentous phages are flexible nucleoprotein rods about 60 Å in diameter and 1–2 μm long. Their circular, single-stranded DNA is encapsulated at the core of a tubular array of protein subunits. Each protein subunit contains only about 50 amino acids and is almost entirely α-helical. Concentrated gels of this phage type can be oriented into fibers that give relatively high-quality X-ray diffraction patterns. The diffraction patterns of the best-known filamentous phages (fd, f1, and M13) are difficult to analyze because there is a perturbation in the helical arrangement of subunits (Marvin et al. 1974b). The similar phage Pf1 gives a less complicated diffraction pattern (Figs. 1 and 2) and is therefore the strain that will be emphasized here.

The filamentous phages have several unique features that enable structure determination without resorting to multiple isomorphous replacement. The technique that we have used is molecular model building, a technique that has been successful for macromolecules such as the α helix and DNA. The word “model” as used here is not a synonym for “hypothesis”; the stereochemical constraints imposed by a precise molecular model are an essential part of this technique of structure determination.

METHODS
Elongated assemblies of subunits such as filamentous phage seldom form true crystals. Instead, they form oriented, semicrystalline fibers in which the assemblies lie parallel to one another and hence diffract X rays in unison. Most elongated assemblies are helical arrangements of identical subunits. There are two periodicities along the axis of a helix: the unit rise...