Structure determination of the bacteriophage MS2

The structure of the bacterial virus MS2 has been solved at 3.3 A resolution. Initial phases to 13 A resolution were obtained from a model based on the known coordinates of the plant virus southern bean mosaic virus. These phases were extended in small steps to a resolution of 3.4 A. The phases obtained represented essentially the Babinet opposite of the true structure and were not of a sufficiently good quality to allow an interpretation of the electron density contoured at negative levels. Difference Fourier maps of two heavy-atom derivatives based on these phases were interpretable, and these derivatives were used to calculate isomorphous replacement phases at 8.8 A resolution. Phase extension to 3.3 A resolution led to maps which could be easily interpreted.