SIR2000, a program for the automatic ab initio crystal structure solution of proteins

A new phasing procedure is described working both in direct and in reciproc al space. The procedure has been implemented into the program SIR2000, the heir to SIR99, and it is able routinely to solve ab initio crystal structur es of proteins without any use of prior information and any user interventi on. The moduli and the flow diagram of SIR2000 are also described and its e fficiency tested on several protein diffraction data sets. Success has been attained for crystal structures with up to almost 2000 non-hydrogen atoms in the asymmetric unit and resolution higher than 1.2 Angstrom. The phasing process is analysed to provide a better insight into the role of the vario us steps of the procedure.