Photosystem II electron transfer in highly herbicide resistant mutants ofSynechocystis6714

Photoheterotrophic growth of unicellular cyanobacterium Synechocystis sp. PCC 6803 gtr− dependent on fructose

Abstract

Although cyanobacteria have specialized for a photolithoautotrophic mode of life during evolution many cyanobacterial strains have been identified as being capable of photoheterotrophy or even chemoheterotrophy. The mutant strain ofSynechocystissp. PCC 6803, which lacks thegtrgene coding for the strain’s glucose/fructose permease, has been believed to be a strict photolithoautotroph in the past as it has lost the wild type’s facility to use external glucose for both photoheterotrophy and light-induced chemoheterotrophy. However, recent experiments revealed the strain’s capacity to use fructose for mixotrophic and photoheterotrophic growth, a sugar which is toxic for the wild type. Both the growth rate and the amount of fructose incorporated into the cells increased along with the fructose concentrations in the surrounding medium. Furthermore an increase of the total carbon mass of the cells within a liquid culture over a period of photoheterotrophic growth could be demonstrated. Contrary to the wild type, glucose could not be used for photoheterotrophic growth, and chemoheterotrophic growth failed with fructose as well as with glucose.

Graphic abstract

Higher plant-like fluorescence induction and thermoluminescence characteristics in cyanobacterium, Spirulina mutant defective in PQH2 oxidation by cytb6/f complex

Characterization of the photosynthetic electron transport in a mutant of Spirulina platensis, generated by chemical mutagenesis, demonstrated that the electron transfer from the plastoquinone (PQ) to cytochrome b6/f was slowed. Thermoluminescence (TL) measurements suggested the presence of reversed energy flow via PQ, which resulted in an emergence of the plant-like after-glow TL band at 45 degrees C that could be enhanced by the transthylakoidal pH gradient and could be eliminated by an uncoupler, FCCP. The localization of the changes in the electron transport of the mutant cells measured by various methods revealed that the re-oxidation of the PQ pool is hampered in the mutant compared to the wild-type cells. The reduction in energy migration was localized between PQ and PS I reaction centers.

Introduction to competition between continuous cultures of Phaeodactylum tricornutum and Dunaliella tertiolecta

When the diatom, Phaeodactylum tricornutum, and the microalga, Dunaliella tertiolecta, are cultured together in a chemostat at dilution factors of approximately 0.5 day-1, the diatom develops the higher population density. At dilution factors above 1.2 day-1 the inability of the diatom to assimilate nutrient as fast as it flows into the chemostat results in the microalga generating the larger population. This change in population densities is accompanied by an increase in the chlorophyll content of the diatom and a decrease in the chlorophyll content of the microalga. Two species of phytoplankton can coexist when they compete for nutrient in a chemostat providing they do not otherwise interact. When the species do interact coexistence in a stable steady state is possible providing intraspecies interactions exceed the interactions between the species. Both species adjust their consumption to minimise the concentration of nutrient in the chemostat and their growth is modified to match the dilution factor of the flow.

Construction and characterization of a functional mutant of Synechocystis 6803 harbouring a eukaryotic PSII-H subunit

A Synechocystis 6803 mutant carrying a chimaeric photosystem II (PSII), in which the Zea mays PsbH subunit (7.7 kDa calculated molecular mass) replaces the cyanobacterial copy (7.0 kDa), was constructed. With the exception of the N-terminal 12 amino acid extension, which has a phosphorylatable threonine, the eukaryotic polypeptide is 78% homologous to its bacterial counterpart. Biochemical characterization of this mutant shows that it expresses the engineered gene correctly and is competent for photoautotrophic growth. Fluorescence analysis and oxygen evolution measurements in the presence of exogenous acceptors indicate that the observed phenotype results from a chimaeric PSII rather than from the absence of function associated with PsbH, suggesting that the heterologous protein is assembled into a functional PSII. Inhibition of oxygen evolution by herbicides belonging to different classes shows that the sensitivity of the mutant PSII is changed only towards phenolic compounds. This result indicates slight conformational modification of the QB/herbicide binding pocket of the D1 polypeptide caused by the bulky PsbH protein in the mutant, and also suggests close structural interaction of the D1 and PsbH subunits in the topological arrangement of PSII.

Thermoluminescence and flash-induced oxygen yield in herbicide resistant mutants of the D1 protein in Synechococcus PCC7942

Several strains of Synechococcus PCC7942 carrying point mutations in the gene psbA were studied by thermoluminescence and polarographic measurement of flash-induced oxygen yield. The following results were obtained: (a) Replacement of Ser-264 in D1 by Ala (mutant Di1) or Gly (mutant G264) resulting in DCMU and atrazine resistance leads to a downshift of the thermoluminescence (TL) B-band peak temperature from 40 degrees C in wild-type thylakoids to about 30 degrees C. In dark adapted samples of both mutants the TL and oxygen yield pattern induced by a train of single turnover flashes were strongly damped indicative of a high miss factor. (b) In contrast to Ser-264 mutants, replacement of Phe-255 in D1 by Tyr (mutant Tyr5) induced strong resistance to atrazine but not to DCMU and did not affect the peak termperature of the B-band and the flash-induced TL and oxygen yield patterns. In this respect mutant Tyr5 resembles the wild type. (c) No significant differences have been found between strains with single site mutations in psbAI and normal psbAII/psbAIII genes, and strains with same mutations in psbAI but additional deletion of psbAII and psbAIII. Obviously in strains were psbAI is present, PS II complexes containing gene products of psbAII and psbAIII are not assembled in detectable amounts. (d) Strains with double mutations at positions 264 and 255 display a downshift of the B-band peak temperature. Their oscillatory patterns of B-band intensity and oxygen yield are highly damped. This behaviour is similar to strains D1 and G264 which are modified at position 264 only. We extend reports on additivity of mutation effects on herbicide binding to binding of QB. (e) Mutations at the QB site not only influence the binding of QB and herbicides but also change the thermoluminescence quantum yield and the lifetimes of the redox states S2 and S3 of the water oxidase. This finding might indicate long ranging effects on Photosystem II exerted by structural modifications of the QB site. From these data we conclude that Ser-264 is essential for binding of atrazine, DCMU and QB, whereas Phe-255 is involved in atrazine binding and its substitution by Tyr does not markedly affect QB or DCMU binding in Synechococcus PCC7942.

Mutation in phenol-type herbicide resistance maps within the psbA gene in Synechocystis 6714

A Synechocytis 6714 mutant resistant to the phenol-type herbicide ioxynil was isolated and characterized. Sensitivity to DCMU and atrazine was tf measured in whole cells and isolated thylakoids. The mutant presents the same sensitivity to atrazine as the wild type and a slightly increased sensitivity to DCMU. A point mutation has been found at codon 266 in the psbAI coding locus (AAC to ACC) resulting in an amino acid change from asparagine to threonine in the D1 protein.

Molecular genetics of herbicide resistance in cyanobacteria

Cyanobacteria offer an excellent model system for studies of herbicide resistance in higher plants. Mutants resistant to classical and non-classical herbicides have been isolated, and in some cases the amino acid alteration(s) are known. Mutations in plants, algae, photosyntehtic bacteria, and cyanobacteria are compared. Data concerning the question of dominance or recessiveness of herbicide resistance in cyanobacteria are also discussed.