Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase

Merohedral twinning is a packing anomaly that seriously impairs the determi nation of macromolecular crystal structures. Crystals of deacetoxycephalosp orin C synthase (DAOCS), an enzyme involved in the expansion of the penicil lin nucleus to form the core structure of the cephalosporin antibiotics, we re found to be merohedrally twinned by many diagnostic criteria. Here, the structure determination of DAOCS from twinned crystals based on a combinati on of isomorphous replacement and the use of a multiple-wavelength diffract ion data set is described.