Study of QB- stabilization in herbicide-resistant mutants from the purple bacterium Rhodopseudomonas viridis

Area of Science:

• Photosynthesis research
• Bacterial photosynthesis
• Electron transport chains

Background:

• Reaction centers in Rhodopseudomonas (Rps.) viridis contain primary donor (P) and quinone acceptors (QA, QB).
• Herbicide resistance in mutants often stems from alterations in the QB binding pocket.
• Understanding these alterations is key to elucidating electron transfer mechanisms.

Purpose of the Study:

• To investigate the pH dependence of charge recombination rates (kBP and kAP) in herbicide-resistant mutants of Rps. viridis.
• To compare these rates to the wild type to understand the impact of specific mutations.
• To correlate structural changes in the QB pocket with functional alterations in electron transfer.

Main Methods:

• Flash-induced absorbance change technique was used to measure rate constants.
• Studies were conducted on chromatophores from three herbicide-resistant mutants (T1, T3, T4) and the wild type.
• pH dependence of rate constants was analyzed.

Main Results:

• Mutant T4 showed pH dependence curves for kBP similar to the wild type, suggesting structural rearrangement, not charge distribution changes, affects DCMU sensitivity.
• Mutant T3 exhibited a 6-fold increase in kAP (4200 s-1 at pH 8) compared to the wild type (720 s-1 at pH 8).
• The mutation Val M263----Phe in T3 is proposed to decrease the free energy between P+QA- and P+I- by ~49 meV, with altered pH dependence of kAP.

Conclusions:

• Mutations in the QB binding pocket significantly impact electron transfer kinetics and pH dependence.
• The Val M263----Phe mutation in T3 alters the QA pocket, affecting charge recombination dynamics.
• Structural changes in the reaction center influence the efficiency and regulation of electron flow.