Maximum-entropy calculation of the electron density at 4 Å resolution of Pf1 filamentous bacteriophage

Abstract

A 4 Å 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 Å long, oriented roughly parallel to the virion axis but slewing by about 1/6th turn while running from a radius of 28 Å to one of 13 Å. Within these rods, there is a helical periodicity with a pitch of 5 to 6 Å. 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.