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Poster: Oxidative Stress

Abs # 131: A photoautotrophic mutant of Chlamydomonas reinhardtii that accumulates b-tocopherol instead of a-tocopherol

Presenter: Sirikhachornkit, Anchalee , anchalee@nature.berkeley.edu
AuthorsSirikhachornkit, Anchalee  (A)   Shin, Jai W (A)   Baroli, Irene  (A)   Niyogi, Krishna  (A)  
Affiliations: (A): University of California

The enzyme 2-methyl-6-phytylbenzoquinone methyltransferase catalyzes the conversion of 2-methyl-6-phytylbenzoquinone (MPBQ) to 2,3-dimethyl-6-phytylbenzoquinone (DMPBQ), a precursor of the antioxidant a-tocopherol. Previous studies of Synechocystis sp. PCC6803 showed that this enzyme also functions in the biosynthesis of plastoquinone, an essential electron carrier in photosynthesis (and respiration in cyanobacteria). Therefore, a null mutation in the gene encoding MPBQ methyltransferase would lead to a lethal phenotype. In the case of maize, plastoquinone-deficient mutants showed an absence of a-tocopherol and a seedling-lethal phenotype. We have isolated a C. reinhardtii mutant that carries a mutation in this gene. HPLC analysis revealed that the mutant completely lacks a-tocopherol and instead accumulates b-tocopherol, showing that the mutation is likely a complete loss-of-function. Interestingly, the mutant was able to grow photoautotrophically, even in high light, indicating that plastoquinone biosynthesis is not blocked by the mutation. This result suggests that C. reinhardtii might possess two separate methyltransferases, one for tocopherol and one for plastoquinone synthesis. This would allow us to study the loss of function of a methyltransferase gene in one pathway without affecting the other pathway.

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