Direct phasing of one-wavelength anomalous-scattering data

This paper presents a brief survey of methods in ab initio phasing of one-wavelength anomalous-scattering data. In particular, the method implemented in the computer program OASIS has been tested using two new data sets from orotidine 5′-monophosphate decarboxylase (OMPDC) [Appleby et al. (2000). Proc. Natl Acad. Sci. USA. In the press] and PurE [Mathews et al. (1999). Structure, 7(11), 1395–1406]. The Se atoms were located by the small-molecule program SAPI. The electron density maps after OASIS and density modification for both structures clearly revealed the Cα trace and, in the case of PurE, most side-chains. The test with the OMPDC data demonstrated that, by exploiting the anomalous signal at a single wavelength, direct methods can be used to determine phases at moderate (∼2.5 Å) macromolecular crystallographic resolution for a large-size protein (5663 non-H atoms in the asymmetric unit). The exceptionally good quality of the electron map shown in the case of PurE suggested that fully automatic model fitting is possible.