Home  About us  Contact
 

Qo Site (qo + site)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Modeling the Qo site of crop pathogens in Saccharomyces cerevisiae cytochrome b

FEBS JOURNAL, Issue 11 2004
Nicholas Fisher
Saccharomyces cerevisiae has been used as a model system to characterize the effect of cytochrome b mutations found in fungal and oomycete plant pathogens resistant to Qo inhibitors (QoIs), including the strobilurins, now widely employed in agriculture to control such diseases. Specific residues in the Qo site of yeast cytochrome b were modified to obtain four new forms mimicking the Qo binding site of Erysiphe graminis, Venturia inaequalis, Sphaerotheca fuliginea and Phytophthora megasperma. These modified versions of cytochrome b were then used to study the impact of the introduction of the G143A mutation on bc1 complex activity. In addition, the effects of two other mutations F129L and L275F, which also confer levels of QoI insensitivity, were also studied. The G143A mutation caused a high level of resistance to QoI compounds such as myxothiazol, axoxystrobin and pyraclostrobin, but not to stigmatellin. The pattern of resistance conferred by F129L and L275F was different. Interestingly G143A had a slightly deleterious effect on the bc1 function in V. inaequalis, S. fuliginea and P. megasperma Qo site mimics but not in that for E. graminis. Thus small variations in the Qo site seem to affect the impact of the G143A mutation on bc1 activity. Based on this observation in the yeast model, it might be anticipated that the G143A mutation might affect the fitness of pathogens differentially. If so, this could contribute to observed differences in the rates of evolution of QoI resistance in fungal and oomycete pathogens. [source]

Role of the N- and C-terminal regions of the PufX protein in the structural organization of the photosynthetic core complex of Rhodobacter sphaeroides

FEBS JOURNAL, Issue 7 2002
Francesco Francia
The core complex of Rhodobacter sphaeroides is formed by the association of the light-harvesting antenna 1 (LH1) and the reaction center (RC). The PufX protein is essential for photosynthetic growth; it is located within the core in a 1 : 1 stoichiometry with the RC. PufX is required for a fast ubiquinol exchange between the QB site of the RC and the Qo site of the cytochrome bc1 complex. In vivo the LH1,PufX,RC complex is assembled in a dimeric form, where PufX is involved as a structural organizer. We have modified the PufX protein at the N and the C-terminus with progressive deletions. The nine mutants obtained have been characterized for their ability for photosynthetic growth, the insertion of PufX in the core LH1,RC complex, the stability of the dimers and the kinetics of flash-induced reduction of cytochrome b561 of the cytochrome bc1 complex. Deletion of 18 residues at the N-terminus destabilizes the dimer in vitro without preventing photosynthetic growth. The dimer (or a stable dimer) does not seem to be a necessary requisite for the photosynthetic phenotype. Partial C-terminal deletions impede the insertion of PufX, while the complete absence of the C-terminus leads to the insertion of a PufX protein composed of only its first 53 residues and does not affect the photosynthetic growth of the bacterium. Overall, the results point to a complex role of the N and C domains in the structural organization of the core complex; the N-terminus is suggested to be responsible mainly for dimerization, while the C-terminus is thought to be involved mainly in PufX assembly. [source]

Mechanisms influencing the evolution of resistance to Qo inhibitor fungicides,,

PEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 9 2002
Ulrich Gisi
Abstract Fungicides inhibiting the mitochondrial respiration of plant pathogens by binding to the cytochrome bc1 enzyme complex (complex III) at the Qo site (Qo inhibitors, QoIs) were first introduced to the market in 1996. After a short time period, isolates resistant to QoIs were detected in field populations of a range of important plant pathogens including Blumeria graminis Speer f sp tritici, Sphaerotheca fuliginea (Schlecht ex Fr) Poll, Plasmopara viticola (Berk & MA Curtis ex de Bary) Berl & de Toni, Pseudoperonospora cubensis (Berk & MA Curtis) Rost, Mycosphaerella fijiensis Morelet and Venturia inaequalis (Cooke) Wint. In most cases, resistance was conferred by a point mutation in the mitochondrial cytochrome b (cyt b) gene leading to an amino-acid change from glycine to alanine at position 143 (G143A), although additional mutations and mechanisms have been claimed in a number of organisms. Transformation of sensitive protoplasts of M fijiensis with a DNA fragment of a resistant M fijiensis isolate containing the mutation yielded fully resistant transformants, demonstrating that the G143A substitution may be the most powerful transversion in the cyt b gene conferring resistance. The G143A substitution is claimed not to affect the activity of the enzyme, suggesting that resistant individuals may not suffer from a significant fitness penalty, as was demonstrated in B graminis f sp tritici. It is not known whether this observation applies also for other pathogen species expressing the G143A substitution. Since fungal cells contain a large number of mitochondria, early mitotic events in the evolution of resistance to QoIs have to be considered, such as mutation frequency (claimed to be higher in mitochondrial than nuclear DNA), intracellular proliferation of mitochondria in the heteroplasmatic cell stage, and cell to cell donation of mutated mitochondria. Since the cyt b gene is located in the mitochondrial genome, inheritance of resistance in filamentous fungi is expected to be non-Mendelian and, therefore, in most species uniparental. In the isogamous fungus B graminis f sp tritici, crosses of sensitive and resistant parents yielded cleistothecia containing either sensitive or resistant ascospores and the segregation pattern for resistance in the F1 progeny population was 1:1. In the anisogamous fungus V inaequalis, donation of resistance was maternal and the segregation ratio 1:0. In random mating populations, the sex ratio (mating type distribution) is generally assumed to be 1:1. Therefore, the overall proportion of sensitive and resistant individuals in unselected populations is expected to be 1:1. Evolution of resistance to QoIs will depend mainly on early mitotic events; the selection process for resistant mutants in populations exposed to QoI treatments may follow mechanisms similar to those described for resistance controlled by single nuclear genes in other fungicide classes. It will remain important to understand how the mitochondrial nature of QoI resistance and factors such as mutation, recombination, selection and migration might influence the evolution of QoI resistance in different plant pathogens. © 2002 Society of Chemical Industry [source]