Phytoene Desaturase, the Essential Target for Bleaching Herbicides

Abstract

Many bleaching herbicides with different core structures inhibit phytoene desaturase (PD), a membrane-bound enzyme in the carotenogenic pathway catalyzing the hydrogen abstraction step at the first C40precursor of β-carotene. Prospects are good that new PD-active herbicides will be discovered by screening for bleaching activity. Accordingly, interest in PD enzymology and molecular genetics has increased. Although active carotenogenic cell-free systems are available, no isolation of PD has been achieved since the enzyme cannot be detected in its isolated form due to complete loss of activity. A portion of theRhodobacterPD gene was incorporated into an appropriate plasmid which could be expressed inE. coli.This system was used to produce an antibody specific against PD from higher plants as well asRhodobacter.All PDs assayed had an apparent molecular weight of 52 to 55 kDa. ARhodobactergene probe hybridized with a 3.1 kbBamH I fragment fromAphanocapsawhich allowed us to sequence the PD gene from this cyanobacterium. Its DNA sequence matched with the apparent molecular weight of the PD band in the western blot, and a fusion-gene product was found to be immunoreactive with theRhodobacterPD antibody,Anacystismutants were produced exhibiting cross-resistance against norflurazon and fluorochloridone. Apparently, this resistance is due to an altered PD with concurrent decrease of inhibitor binding affinity. Cloning of the resistant gene into the wild type is in progress.