Conservation of NADPH utilization by chorismate synthase and its implications for the evolution of the shikimate pathway.

Summary The shikimate pathway is essential for the biosynthe- sis of aromatic compounds. The seventh and last step is catalysed by chorismate synthase, which has an absolute requirement for reduced FMN in its active site. There are two classes of this enzyme, which are distinguished according to the origin of the reduced cofactor. Monofunctional chorismate synthases se- quester it from the cellular environment whereas bifunctional enzymes can generate reduced FMN at the expense of NADPH. These bifunctional enzymes are found in fungi and the ciliated protozoan Euglena gracilis while all bacterial and plant enzymes are monofunctional. In this study, we introduce an in vivo screen, which is based on a chorismate synthase- deficient Saccharomyces cerevisiae strain, allowing the classification of hitherto uncharacterized choris- mate synthases. This analysis revealed that bifunc- tionality is present in the enzymes of protozoan species. In contrast, all bacterial and plant enzymes tested are monofunctional. In addition, we demon- strate that a monofunctional chorismate synthase confers prototrophy in conjunction with a NADPH : FMN oxidoreductase indicating that bifunctionality is required due to the lack of free reduced FMN in fungal and possibly protozoan species. Interestingly, the distribution of bifunctional chorismate synthase concurs with the presence of a pentafunctional enzyme complex.