2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MECDP) synthase from Salmonella typhimurium, ispF gene product, takes part in biosynthesis of the isomers isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate, which are universal five-carbon precursors of isoprenoids. The enzyme is involved in the non-mevalonate pathway of isoprenoid synthesis which is essential in all organisms. The non-mevolanate route is used by many bacteria and human pathogens. This route appears to involve seven enzymes. MECDP synthetases, ispF and YgbB proteins, catalyze the fifth stage of the pathway. They convert 4-diphosphocytidyl-2C-methyl-D-erythritol-2-phosphate into MECDP and CMP. The enzyme is a homotrimer with three active sites located in a cleft lined by residues from two subunits. Each active site binds a Zn2+ ion and the ribose and diphosphate of CDP. The non-mevalonate pathway is present in many bacteria, some algae, certain protozoa, and in the plastids of higher plants, but not in mammals or archaea. Therefore, these enzymes have been recognised as promising drug targets.