Plant Physiol.
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Plant Physiology 94:614-620 (1990)
© 1990 American Society of Plant Biologists

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Metabolism and Enzymology

Acetohydroxy Acid Synthase Activity in Chlorella emersonii under Auto- and Heterotrophic Growth Conditions 1

Dorit Landstein, David M. Chipman, Shoshana (Malis) Arad and Ze've Barak

Department of Biology, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel, The Institutes for Applied Research, Ben-Gurion University of the Negev, P.O.B. 1025, Beer-Sheva 84110, Israel

Acetohydroxyacid synthase (AHAS) activity was studied in the green unicellular alga Chlorella emersonii. This activity and its regulation was compared in the algae grown autotrophically and heterotrophically on glucose in the dark. No evidence for the existence of more than one enzyme was found. The activity in crude extracts from either heterotrophically or autotrophically grown cells showed a Km for pyruvate of 9 millimolar, a 22-fold preference for 2-ketobutyrate over pyruvate as the second substrate, 50% inhibition by 0.5 millimolar valine, and 50% inhibition by 0.3 micromolar sulfometuron methyl (SMM). Spontaneous mutants of the alga resistant to SMM were isolated, which appeared to be single gene mutants containing SMM-resistant AHAS activity. Hence, AHAS appears to be the sole direct target site of SMM in C. emersonii. The fact that the mutants had equivalent SMM resistance under auto- and heterotrophic conditions further supports the conclusion that the same enzyme functions under both physiological regimes. The addition of valine and isoleucine leads to partial relief of SMM inhibition of biomass increase, but not of SMM inhibition of cell division.

1 This research was supported in part by grant No. 86-00205 from the U.S.-Israel Binational Science Foundation (to Z. B. and D. M. C.) and was performed in partial fulfillment of the requirements for the Ph.D. degree of D.L.






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