A rapid procedure for selective enrichment of photosynthetic electron transport mutants.

Abstract

Metronidazole (2-methyl-5-nitroimidazole-1-ethanol) is effective for the selective enrichment of mutants of Chlamydomonas reinhardtii that possess impaired photosynthetic electron transport. More than 99.9% of wild-type cells are killed when incubated in the presence of 6-10 mM metronidazole for 24 h under illumination of 7500 lux. Survival of wild-type cells in darkness and of mutants that are blocked at different steps in photosynthetic electron transport is nearly 100% when incubated in the presence of the drug under identical conditions. The toxicity of metronidazole depends on its reduction by photosynthetic electron transport. Light-dependent O2 uptake mediated by metronidazole is shown to require active photosystem I in vitro and in vivo. Ferredoxin is necessary for metronidazole reduction by thylakoid membrane fractions enriched in photosystem I activity. Probably the toxicity of metronidazole is due to the formation of lethal derivatives of the drug or to the accumulation of H2O2, which could occur on autooxidation of metronidazole reduced by 1 electron. Apparently mutants of C. reinhardtii, and probably other photosynthetic organisms, with any lesion in photosynthetic electron transport from the oxidizing side of photosystem II to ferredoxin can be isolated by metronidazole treatment of mutagenized cultures.