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The filamentous bacterial virus Pf1 is a simple model for biological filaments. We have studied the structure of the virion and report here that the helix parameters of Pf1 change sharply with temperature at about 8 degrees C. Local interactions between protein subunits change by only a few tenths of an angstrom, but the changes are amplified between one end and the other of the virion to a rotation of 15 turns and a translation of 1,000 A. The limited nature of the phase transition is probably due to the constraints of 'knobs-into-holes' interaction between side chains of adjacent alpha-helical protein subunits. Treatment of the virion with ether causes a rearrangement of protein subunits into sheets, with the alpha-helices normal to the plane of the sheet. This phase transition suggests a model for virion assembly in the bacterial membrane.

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The paper describes the use of computer generated microfilm masks and briefly discusses their advantages over more traditional methods of preparation.

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