The hand of the filamentous bacteriophage helix

Filamentous bacteriophage (Inovirus) is a widely studied model system in molecular biophysics. The structure of the virion has been analysed by various methods, but the methods have seldom questioned the hand of the virion helix. The hand of the helix relating the protein subunits in the class II virus strain Pf1 was chosen by calculating an electron-density distribution from X-ray fibre diffraction data, using a maximum-entropy method, but to our knowledge this method has not been used for a similar purpose in any other system. Moreover, this same hand was extended only by analogy, with no direct analysis of the corresponding data, to the class I virus strain Ff (fd, f1, M13), which has a different helix symmetry. Here we use published solid-state NMR data to confirm the validity of the hand of Pf1 chosen by the maximum-entropy method, and to confirm the extension to Ff.