Structure determination of bacteriophage PP7 from Pseudomonas aeroginosa: from poor data to a good map

The structure of bacteriophage PP7 from Pseudomomas aeruginosa was determin ed to 3.7 Angstrom resolution. Triclinic crystals of three forms were obtai ned, diffracting to between 4.5 and 3.4 Angstrom resolution. The quality of the crystals was exceptionally poor, leading to problems in the evaluation of the recorded images and to a final data set which would appear to be us eless with standard criteria for protein crystals, In all crystal forms, th e unit cell contains two icosahedral particles, providing 120-fold non-crys tallographic symmetry. For two of the crystal forms, the particle orientati ons were calculated using the self-rotation function. The two particles in the asymmetric unit had very similar but distinct orientations, The positio n of the second particle was found using the Patterson function. Initial ph ases to 15 Angstrom resolution were calculated using the related phage MS2 as a model. Real-space averaging was performed and phases were extended fro m 15 Angstrom resolution to the limit of the data. The map was improved sig nificantly by using only the 'high' resolution data in the resolution range 7-3.7 Angstrom, allowing the positions of most side chains to be determine d. The better quality of the 7-3.7 Angstrom resolution map is presumably a consequence of the presence of satellite crystals. The position of the seco nd particle was improved using the correlation coefficient in the averaging process to monitor the refinement by moving the particle around in small s teps.