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A 4 A electron-density map of Pf1 filamentous bacterial virus has been calculated from x-ray fiber diffraction data by using the maximum-entropy method. This method produces a map that is free of features due to noise in the data and enables incomplete isomorphous-derivative phase information to be supplemented by information about the nature of the solution. The map shows gently curved (banana-shaped) rods of density about 70 A long, oriented roughly parallel to the virion axis but slewing by about 1/6th turn while running from a radius of 28 A to one of 13 A. Within these rods, there is a helical periodicity with a pitch of 5 to 6 A. We interpret these rods to be the helical subunits of the virion. The position of strongly diffracted intensity on the x-ray fiber pattern shows that the basic helix of the virion is right handed and that neighboring nearly parallel protein helices cross one another in an unusual negative sense.Two structural classes of filamentous bacterial viruses have been identified (1). Class 1, exemplified by the fd (= fl = M 13) species, has been extensively studied by biochemical and genetic techniques whereas class 2, exemplified by the Pfl species, gives higher resolution x-ray fiber diffraction data and has been intensively studied by structural techniques (reviewed in refs. 2 and 3). Both classes of virion have a cylindrical protein coat of 60 A average outer diameter and 20 A average inner diameter encapsulating a single-stranded circular DNA molecule that stretches down the length of the central hole. The protein coat is essentially a helical array of several thousand identical protein units, although there are minor proteins at the ends of the virion that make up about 1% by weight of the coat.

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X-ray diffraction analysis of connective tissue samples, which contain type I and type III collagen shows that twisted collagen fibrils are a general principle of assembly. The occurrence of twisted fibrils in native wet Chordae tendineae, skin and Aorta is combined with a shorter axial periodicity of about 65 nm. This shorter D period is shown to be directly related to the tilt of the molecules, which have to be curved to build-up twisted fibrils.