The first committed step in the biosynthetic pathway of carotenoids in plants and algae is the conversion of geranylgeranyl pyrophosphate (GGPP) to prephytoene pyrophosphate (PPPP), which is converted to phytoene. We have cloned the gene pys that encodes the enzyme phytoene synthase in the cyanobacterium Synechococcus PCC7942. The co-expression of pys in cells of Escherichia coli together with the gene crtE from Erwinia uredovora, which encodes geranylgeranyl pyrophosphate synthase, resulted in accumulation of phytoene. This result indicates that phytoene synthase is a single polypeptide enzyme that catalyzes the 2-step reaction from GGPP to phytoene. The deduced amino acid sequence of pys is highly conserved with that of pTOM5, a tomato cDNA that is differentially expressed during fruit ripening. These findings suggest that pTOM5 encodes phytoene synthase in tomato.
