Molecular and biochemical characterization of herbicide-resistant mutants of cyanobacteria reveals that phytoene desaturation is a rate-limiting step in carotenoid biosynthesis.

Mutant strains of the cyanobacterium Synechococcus sp. PCC 7942 that are resistant to the herbicides norflurazon and fluridone were analyzed. These herbicides inhibit phytoene desaturase, a key enzyme in the carotenoid biosynthetic pathway. In three mutants the phenotype was related to specific point mutations in pds, the gene encoding phytoene desaturase. Since the resistance was manifested in a cell-free carotenogenic assay, it is evident that the predicted amino acid changes in the target protein alter the enzyme-herbicide interactions. In addition, the mutations also reduced the in vitro activity of phytoene desaturase. Increased levels of the substrate phytoene, accompanied by a reduction in colored carotenoids, were detected in cells of each of the mutant strains. A correlation was established between the degree of increase in the steady-state levels of phytoene and the extent of reduction in total carotenoid content in the cells. These two phenomena in turn are correlated with the rate of enzymatic activity of phytoene desaturase that was measured in vitro. Hence we deduce that phytoene desaturation is a rate-limiting step in carotenogenesis in cyanobacteria. Support for this conclusion is obtained from analysis of cells of an additional mutant strain, which overexpress phytoene desaturase due to a deletion mutation in the promoter region of pds. Cells of this mutant contained more colored carotenoids than the wild-type and were resistant to herbicides that inhibit phytoene desaturase.