Herbicide resistance in Chlamydomonas reinhardtii results from a mutation in the chloroplast gene for the 32-kilodalton protein of photosystem II

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Abstract
The isolation and characterization of a uniparental mutant of C. reinhardtii that is resistant to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) are reported. Such herbicides inhibit photosynthesis by preventing transfer of electrons in photosystem II from the primary stable electron acceptor Q to the secondary stable electron acceptor complex B, which is thought to contain a protein of 32kDa [kilodaltons] and a bound quinone. Herbicide binding to the 32-kDa protein probably alters the B complex so that electron transfer from Q is prohibited. Whole and broken-cell preparations of the mutant alga show a resistance to the effects of herbicide on electron transfer from Q to B, as measured by fluorescence-induction kinetics. In the absence of herbicide, mutant cells exhibit a slower rate of Q to B electron transfer than do wild-type cells. The 32-kDa protein from wild-type cells, but not mutant cells, binds azido[14C]atrazine at 0.1 .mu.M. psbA, the chloroplast gene for the 32-kDa protein, was isolated from wild-type and herbicide-resistant algae and the coding regions of the gene that are contained in 5 exons were sequenced. The only difference between the exon nucleotide sequences of the wild-type and mutant psbA is a single T-A to G-C transversion. This mutation results in a predicted amino acid change of serine in the wild-type protein to alanine in the mutant. This alteration in the 32-kDa protein may be the molecular basis for herbicide resistance in the C. reinhardtii mutant.