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

Chloroplast transformation in Chlamydomonas with high velocity microprojectiles

Bombardment of three mutants of the chloroplast atpB gene of Chlamydomonas reinhardtii with high-velocity tungsten microprojectiles that were coated with cloned chloroplast DNA carrying the wild-type gene permanently restored the photosynthetic capacity of the algae. In most transformants of one of the mutants, a fragment with a 2.5-kilobase deletion was restored to normal size by a homologous replacement event; in about 25 percent of the transformants the restored restriction fragment was 50 to 100 base pairs smaller or larger than that of wild type. About one-fourth of the transformants of this mutant contained unintegrated donor plasmid when first examined. This plasmid persisted in four different transformants after 65 cell generations of continuous liquid culture but was lost from all transformants maintained on plates of selective medium. The restored wild-type atpB gene remains in all transformants as an integral part of the chloroplast genome and is expressed and inherited normally.

The psbA gene of DCMU-resistant Euglena gracilis has an amino acid substitution at serine codon 265

The psbA gene coding for the herbicide binding QB protein of photosystem II has been sequenced previously (Karabin et al. 1984). A herbicide resistant mutant of Euglena, Euglena gracilis ZR, was studied by sequencing part of the psbA gene and its corresponding mRNA. Sequencing reactions were done by annealing a psbA specific, end-labeled DNA-oligomer to total chloroplast DNA or RNA and extending this primer with reverse transcriptase in the presence of the four dideoxynucleotides. An amino acid substitution from serine to alanine at position 265 was detected. All known herbicide resistant higher plants sequenced to date and the Chlamydomonas mutant DCMU-4 show a change at exactly this same position, but the substitution in higher plants is from serine to glycine.

Atrazine Resistance in Chenopodium album: Low and High Levels of Resistance to the Herbicide Are Related to the Same Chloroplast PSBA Gene Mutation

In Chenopodium album two different levels of atrazine resistance have been found according to following criteria: lethal dose and leaf fluorescence curve. The intermediate (I) phenotype is represented by a low level of resistance and a typical I fluorescence curve. It arose at high frequency, within one generation, after self-pollination of particular plants displaying a susceptible (S) phenotype. The resistance phenotype (Ri) has a high level of resistance and presents a typical resistant fluorescence curve. It appeared after self-pollination of chemically treated I plants. The I, Ri, and also R (resistant plants found in atrazine treated fields) phenotypes contain a serine to glycine mutation at amino acid position 264 in the chloroplast psbA gene product. The steady state level of the psbA gene transcript is not modified between S, I, Ri, and R phenotypes.

Interaction of Herbicides and Quinone with the Q(B)-Protein of the Diuron-Resistant Chlamydomonas reinhardtii Mutant Dr2

We have used the diuron-resistant Dr2 mutant of Chlamydomonas reinhardtii which is altered in the 32 kilodalton Q(B)-protein at amino acid 219 (valine to isoleucine), to investigate the interactions of herbicides and plastoquinone with the 32 kilodalton Q(B)-protein. The data contained in this report demonstrate that the effects of this mutation are different from those of the more completely characterized mutant which confers extreme resistance to triazines in higher plants. The mutation in C. reinhardtii Dr2 confers only slight resistance to a number of inhibitors of photosynthetic electron transport. Extreme triazine resistance results from an increase in the binding constant of the herbicide with the 32 kilodalton Q(B)-protein, in contrast the diuron binding constant for chloroplasts isolated from wild-type (sensitive) Chlamydomonas and the resistant Dr2 are indistinguishable. We conclude that the altered structure in the 32 kilodalton Q(B)-protein of Dr2 does not directly affect the diuron binding site. This mutation appears to alter the steric properties of the binding protein in such a way that diuron and plastoquinone do not directly compete for binding. This steric perturbation confers mild resistance to other herbicidal inhibitors of photosynthesis and alters the kinetics of Q(A) to Q(B) electron transfer.

Characterization of photosystem II mutants of Chlamydomonas reinhardii lacking the psbA gene

We have examined 78 chloroplast mutants of Chlamydomonas reinhardii lacking photosystem II activity. Most of them are unable to synthesize the 32 Kdalton protein. Analysis of 22 of these mutants reveals that they have deleted both copies of the psbA gene (which codes for the 32 Kdalton protein) in their chloroplast genome. Although these mutants are able to synthesize and to integrate the other photosystem II polypeptides in the thylakoid membranes, they are unable to assemble a stable functional photosystem II complex. The 32 Kprotein appears therefore to play an important role not only in photosystem II function, but also in stabilizing this complex.

Multidisciplinary research in photosynthesis: A case history based on the green alga Chlamydomonas

This article examines the contribution of a unicellular green alga Chlamydomonas to progress in photosynthetic research. The objective is to focus on the aspects of Chlamydomonas that have provided an advantage over other photosynthetic organisms in investigating photosynthesis. To do this we discuss several examples that demonstrate the progress from a genetic study to a multidisciplinary approach that probes higher levels of complexity within the organism. These examples include the function and molecular regulation of electron transport components between photosystem II and photosystem I, the molecular genetics of the herbicide binding protein of photosystem II, and several different studies that have derived from a search for rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) mutants in Chlamydomonas, including chloroplast ribosome function, the regulation of the large subunit of rubisco, and the interaction between photosynthetic electron transport and carbon metabolism.

Inhibitors of photosystem II and the topology of the herbicide and QB binding polypeptide in the thylakoid membrane

The folding through the thylakoid membrane of the D-1 herbicide binding polypeptide and of the homologous D-2 subunit of photosystem II is predicted from comparison of amino acid sequences and hydropathy index plots with the folding of the subunits L and M of a bacterial photosystem. As the functional amino acids involved in Q and Fe binding in the bacterial photosystem of R. viridis, as indicated by the X-ray structure, are conserved in the homologous D-1 and D-2 subunits of photosystem II, a detailed topology of the binding niche of QB and of herbicides on photosystem II is proposed. The model is supported by the observed amino acid changes in herbicide tolerant plants and algae. These changes are all in the binding domain on the matrix side of the D-1 polypeptide, and turn out to be of functional significance in the QB binding.New inhibitors of QB function are described. Their chemical structure, i.e. pyridones, quinolones, chromones and benzodiones, contains the features of the phenolic type herbicides. Their essential elements, π-charges at particular atoms, QSAR and steric requirements for optimal inhibitory potency are discussed and compared with the "classical" herbicides of the urea/triazine type.

Mutation to herbicide resistance maps within the psbA gene of Anacystis nidulans R2

A psbA gene encoding the target of photosystem II herbicide inhibition, the 32,000-dalton thylakoid membrane protein, has been cloned from a mutant of Anacystis nidulans R2, which is resistant to 3-(3,4-dichlorophenyl)-1,1-dimethylurea-(diuron). A cloned DNA fragment from within the coding region of this gene transforms wild-type cells to herbicide resistance, proving that mutation within psbA is responsible for that phenotype. The mutation consists of a single nucleotide change that replaces serine at position 264 of the wild-type protein with alanine in that of the diuron-resistant mutant.

Chloroplast-coded atrazine resistance in Solanum nigrum: psbA loci from susceptible and resistant biotypes are isogenic except for a single codon change

The 32-kDa photosystem II protein of the chloroplast is thought to be a target molecule for the herbicide atrazine. The psbA gene coding for this protein was cloned from Solanum nigrum atrazine-susceptible ('S') and atrazine-resistant ('R') biotypes. The 'S' and 'R' genes are identical in nucleotide sequence except for an A to G transition, predicting a Ser to Gly change at codon 264. The same predicted amino acid change in psbA was previously shown for an Amaranthus hybridus 'S' and 'R' biotypes which had, in addition, two silent nucleotide changes between the genes (Hirschberg, J. and McIntosh, L., Science 222, 1346-1349, 1983). Occurrence of the identical, non-silent change in psbA in different 'S' and 'R' weed biotype pairs suggests a functional, herbicide-related role for this codon position.

Chloroplast promoter driven expression of the chloramphenicol acetyl transferase gene in a cyanobacterium

The putative promoter region of the chloroplast encoded ps2B gene (the gene encoding the 32kD herbicide binding B protein of photosystem II (1-4)) has been fused to a chloramphenicol acetyl transferase (CAT) gene that lacks its bacterial promoter and found to accurately initiate transcription from this promoter when introduced into the cyanobacterium, Anacystis nidulans R2 (or into E. coli). The chloroplast promoter-CAT fusion was introduced into the cells on a plasmid that contains plasmid replication origins for E. coli and Anacystis as well as a second antibiotic resistance marker. Cells transformed with corresponding vectors lacking the promoter region do not express CAT.

Primary structure of the L subunit of the reaction center from Rhodopseudomonas sphaeroides

The reaction center is an integral membrane protein that, together with several cofactors, mediates the primary photochemical events in bacterial photosynthesis. The amino-terminal sequences of the three subunits, L, M, and H, of the reaction center protein and the sequence of the structural gene encoding the M subunit have been reported previously. In the present study, we found that the 3' end of the structural gene encoding the L subunit overlaps by eight bases the 5' end of the gene encoding the M subunit. The primary structure of the L subunit has been determined from the nucleotide sequence of the gene and from analyses of the amino and carboxyl termini of the protein. The sequences of a number of tryptic and chymotryptic peptides were used to corroborate the nucleotide sequence. The L subunit was found to be composed of 281 amino acids (Mr 31,319) and to contain five hydrophobic segments. It is homologous to the M subunit and to a plant thylakoid protein referred to as the QB or Mr 32,000 protein.