MOLECULAR AND BIOCHEMICAL-CHARACTERIZATION OF HERBICIDE-RESISTANT MUTANTS OF CYANOBACTERIA REVEALS THAT PHYTOENE DESATURATION IS A RATE-LIMITING STEP IN CAROTENOID BIOSYNTHESIS
D. Chamovitz et al., MOLECULAR AND BIOCHEMICAL-CHARACTERIZATION OF HERBICIDE-RESISTANT MUTANTS OF CYANOBACTERIA REVEALS THAT PHYTOENE DESATURATION IS A RATE-LIMITING STEP IN CAROTENOID BIOSYNTHESIS, The Journal of biological chemistry, 268(23), 1993, pp. 17348-17353
Mutant strains of the cyanobacterium Synechococcus sp. PCC 7942 that a
re resistant to the herbicides norflurazon and fluridone were analyzed
. These herbicides inhibit phytoene desaturase, a key enzyme in the ca
rotenoid biosynthetic pathway. In three mutants the phenotype was rela
ted to specific point mutations in pds, the gene encoding phytoene des
aturase. Since the resistance was manifested in a cell-free carotenoge
nic assay, it is evident that the predicted amino acid changes in the
target protein alter the enzyme-herbicide interactions. In addition, t
he mutations also reduced the in vitro activity of phytoene desaturase
. Increased levels of the substrate phytoene, accompanied by a reducti
on in colored carotenoids, were detected in cells of each of the mutan
t strains. A correlation was established between the degree of increas
e in the steady-state levels of phytoene and the extent of reduction i
n total carotenoid content in the cells. These two phenomena in turn a
re correlated with the rate of enzymatic activity of phytoene desatura
se that was measured in vitro. Hence we deduce that phytoene desaturat
ion is a rate-limiting step in carotenogenesis in cyanobacteria. Suppo
rt for this conclusion is obtained from analysis of cells of an additi
onal mutant strain, which overexpress phytoene desaturase due to a del
etion mutation in the promoter region of pds. Cells of this mutant con
tained more colored carotenoids than the wild-type and were resistant
to herbicides that inhibit phytoene desaturase.