Although it is clear that acetohydroxy acid synthase (AHAS; EC 126.96.36.199) is the target for sulfonylurea herbicides such as sulfometuron methyl (SMM), there is considerable uncertainty as to the mechanism(s) by which inhibition of AHAS inhibits or kills cells. We have further studied the mode of action of SMM, and its effects on metabolism and physiology in the unicellular green alga Chlorella emersonii var. emersonii. Addition of SMM to cells synchronized to a cycle of 16 h light-8 h dark showed that they were very sensitive to SMM toxicity in the first 16 h of the cell cycle, during which cell mass, protein and DNA increased. The increase in protein, DNA and chlorophyll was halted rapidly after SMM addition. Sulfometuron methyl prevented cell division even if added late in the light stages, when most of the protein and DNA were already synthesized, but did not affect cell division and autospore release if added after protein and DNA synthesis were complete. This suggests that SMM interferes with processes involved in preparation for division, beyond what would be expected if the cells were starved of the branched-chain amino acids needed as precursors for synthesis of proteins in general. The accumulation of α-ketobutyrate (αKB) in the cells in response to addition of SMM, and its possible role in the growth inhibition, was also investigated (in continually illuminated cultures). Intracellular αKB accumulated rapidly within 30 min of SMM addition, but declined nearly to basal levels in several hours. This paralleled the decrease and subsequent recovery of extractable AHAS activity. Despite this, growth of the algal culture did not recover. We suggest that metabolites formed by misincorporation of αKB in place of α-ketoisovalerate (e.g., in the ketopantoate hydroxymethyl transferase reaction) might be responsible for the persistence of growth inhibition. We note that an important difference between the effect of SMM and that observed with externally added αKB is that the ratio between intracellular αKB and α-ketoisovalerate is expected to be high in the first case, but not necessarily in the second.
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acetohydroxy acid synthase
branched-chain amino acids
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This research was supported in part by grant 338/92 from the Israel Science Foundation. It was also supported by the Lily and Sidney Oelbaum Chair in Applied Biochemistry, of which D.M.C. is the incumbent.
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Landstein, D., Epelbaum, S., Arad, S.(. et al. Metabolic response of Chlorella emersonii to the herbicide sulfometuron methyl. Planta 197, 219–224 (1995). https://doi.org/10.1007/BF00202640
- Acetohydroxy acid synthase
- Acetolactate synthase
- Synchronized growth