Molecular Analysis of Carotenoid Biosynthesis

We have cloned the genes pys, encoding phytoene synthase (PYS), and pds, encoding phytoene desaturase (PDS), from cyanobacteria, algae and tomato. Cells of E. coil that express pys could convert geranylgeranyl pyrophosphate to phytoene and cells that express pds could convert phyto- ene to E-carotene. These results suggest that PY8 and PDS are single- polypeptide enzymes that can carry out their catalytic functions in a trans- genic organism that normally does not produce carotenoids. PDS carries out the two dehydrogenation reactions of phytoene. Several norflurazon or fluridone resistant strains of Synechococcus PCC7942 were isolated. Point mutations that lead to amino acid substitutions in PDS were found to confer herbicide resistance in these mutants. Alteration in the kinetics of the enzyme as a result of modifications in PDS, caused reduction in carotenoid synthesis in-vivo, suggesting that PDS is a rate-limiting step in the pathway. Over- expression of pys and pals in Synechococcus PCC7942 resulted in increase of carotenoid content in the cells. Expression of the single pds gene found in tomato is up-regulated at the transcriptional level during fruit ripening.